fix(userspace/libsinsp): user id 0 or group id 0 default values

[FedeDP]

What type of PR is this?

/kind bug

Any specific area of the project related to this PR?

/area libsinsp
/area tests

Does this PR require a change in the driver versions?

What this PR does / why we need it:

Fixed a possible case were the root, /root default values for uid 0 users and gid 0 groups were not set (after #2165).
Moreover, properly use different static members for threadinfo::get_user() and threadinfo::get_loginuser.
Also, added tests to avoid future failures.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[github-actions]

Perf diff from master - unit tests

     5.12%     -1.66%  [.] sinsp_parser::process_event
    11.13%     -1.11%  [.] sinsp_parser::reset
     3.62%     +0.91%  [.] sinsp_evt::load_params
     1.15%     +0.69%  [.] std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>
     6.08%     -0.66%  [.] sinsp::next
     4.43%     -0.55%  [.] gzfile_read
     4.04%     +0.52%  [.] next
     0.15%     +0.46%  [.] sinsp_threadinfo::set_cgroups
     0.55%     -0.41%  [.] sinsp_container_manager::resolve_container
     0.20%     +0.41%  [.] std::_Hashtable<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, libsinsp::state::dynamic_struct::field_info>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, libsinsp::state::dynamic_struct::field_info> >, std::__detail::_Select1st, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true> >::_M_insert_unique_node

Heap diff from master - unit tests

peak heap memory consumption: -1.94K
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

Heap diff from master - scap file

peak heap memory consumption: 0B
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

Benchmarks diff from master

Comparing gbench_data.json to /root/actions-runner/_work/libs/libs/build/gbench_data.json
Benchmark                                                         Time             CPU      Time Old      Time New       CPU Old       CPU New
----------------------------------------------------------------------------------------------------------------------------------------------
BM_sinsp_split_mean                                            +0.0335         +0.0335           145           150           145           150
BM_sinsp_split_median                                          +0.0338         +0.0338           145           150           145           150
BM_sinsp_split_stddev                                          +1.2139         +1.2093             1             1             1             1
BM_sinsp_split_cv                                              +1.1421         +1.1377             0             0             0             0
BM_sinsp_concatenate_paths_relative_path_mean                  +0.0003         +0.0003            61            61            61            61
BM_sinsp_concatenate_paths_relative_path_median                +0.0011         +0.0011            61            61            61            61
BM_sinsp_concatenate_paths_relative_path_stddev                -0.0921         -0.0914             0             0             0             0
BM_sinsp_concatenate_paths_relative_path_cv                    -0.0924         -0.0917             0             0             0             0
BM_sinsp_concatenate_paths_empty_path_mean                     -0.0002         -0.0002            25            25            25            25
BM_sinsp_concatenate_paths_empty_path_median                   -0.0014         -0.0014            25            25            25            25
BM_sinsp_concatenate_paths_empty_path_stddev                   +0.5292         +0.5300             0             0             0             0
BM_sinsp_concatenate_paths_empty_path_cv                       +0.5294         +0.5303             0             0             0             0
BM_sinsp_concatenate_paths_absolute_path_mean                  +0.0409         +0.0409            63            66            63            66
BM_sinsp_concatenate_paths_absolute_path_median                +0.0606         +0.0606            62            66            62            66
BM_sinsp_concatenate_paths_absolute_path_stddev                -0.0185         -0.0185             2             2             2             2
BM_sinsp_concatenate_paths_absolute_path_cv                    -0.0571         -0.0571             0             0             0             0
BM_sinsp_split_container_image_mean                            +0.0054         +0.0054           394           396           394           396
BM_sinsp_split_container_image_median                          +0.0063         +0.0063           394           396           394           396
BM_sinsp_split_container_image_stddev                          -0.0589         -0.0590             3             3             3             3
BM_sinsp_split_container_image_cv                              -0.0639         -0.0640             0             0             0             0

[codecov]

Codecov Report

Attention: Patch coverage is 98.90110% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.16%. Comparing base (1c4698b) to head (451a3d9).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
userspace/libsinsp/sinsp.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2177      +/-   ##
==========================================
+ Coverage   75.07%   75.16%   +0.09%     
==========================================
  Files         256      257       +1     
  Lines       33659    33736      +77     
  Branches     5752     5755       +3     
==========================================
+ Hits        25268    25359      +91     
+ Misses       8391     8377      -14     

Flag	Coverage Δ
libsinsp	75.16% <98.90%> (+0.09%)	⬆️

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[FedeDP]

Cleaned up unused flag.

[FedeDP]

/milestone 0.20.0

[Andreagit97]

/approve

[poiana]

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