Add gRPC testsuite

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
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• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?
Draft of a gRPC test-suite.
There are helper functions to:

• create a test environment
• download and extract artifacts (not currently used in the gRPC testing but I had these functions lingering in my local git so I decided to include them anyway)
• run the arduino-cli (it is confined in the environment as we do in our integration tests)
• run the arduino-cli as a daemon (the cli daemon is started in the background and commands may be sent via gRPC)
• the output is partially colored/decorated

I've put everything inside the internal package, but I see that some of those objects may be part of a more general-purpose library (like the testsuite.Environment object may be potentially reused in other projects.

This implementation has some overlap with our current integration tests in python. As I see it, it has the potential to completely replace pytest if we properly expand it.

Does this PR introduce a breaking change, and is titled accordingly?
No

[cmaglie]

https://github.com/arduino/arduino-cli/runs/7356179761?check_suite_focus=true#step:7:1328 this is an real bug that the testsuite is triggering

[codecov]

Codecov Report

Merging #1806 (757fa06) into master (2dd8976) will increase coverage by 0.77%.
The diff coverage is 79.19%.

@@            Coverage Diff             @@
##           master    #1806      +/-   ##
==========================================
+ Coverage   35.50%   36.27%   +0.77%     
==========================================
  Files         230      232       +2     
  Lines       19532    19497      -35     
==========================================
+ Hits         6934     7073     +139     
+ Misses      11773    11594     -179     
- Partials      825      830       +5     

Flag	Coverage Δ
unit	36.27% <79.19%> (+0.77%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/integrationtest/arduino-cli.go	79.19% <79.19%> (ø)
arduino/cores/packagemanager/install_uninstall.go	9.96% <0.00%> (-5.12%)	⬇️
arduino/cores/packagemanager/download.go	22.89% <0.00%> (-2.11%)	⬇️
cli/update/update.go	0.00% <0.00%> (ø)
commands/core/install.go	0.00% <0.00%> (ø)
commands/core/upgrade.go	0.00% <0.00%> (ø)
commands/core/download.go	0.00% <0.00%> (ø)
commands/daemon/daemon.go	0.00% <0.00%> (ø)
commands/core/uninstall.go	0.00% <0.00%> (ø)
commands/bundled_tools.go
... and 4 more

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[cmaglie]

This PR has currently evolved:

• we internally decided to port the integration tests from python to golang and remove dependencies from python: this task will be done after this PR incrementally over time
• some of the generic utility functions have been moved into a dedicated library in https://go.bug.st/testsuite

[umbynos]

Also would bettere IMHO to use a different name for this variable. And a more clear comment here

[umbynos]

[silvanocerza]

[per1234]

Excellent work Cristian. Thanks!