Merge 183 tests from test_btf into test_progs framework to be exercised

regularly. All the test_btf tests that were moved are modeled as proper
sub-tests in test_progs framework for ease of debugging and reporting.

No functional or behavioral changes were intended, I tried to preserve
original behavior as much as possible. E.g., `test_progs -v` will activate
"always_log" flag to emit BTF validation log.

The only difference is in reducing the max_entries limit for pretty-printing
tests from (128 * 1024) to just 128 to reduce tests running time without
reducing the coverage.

Example test run:

  $ sudo ./test_progs -n 8
  ...
  #8 btf:OK
  Summary: 1/183 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
v2->v3:
 - made pprint use smaller max_entries (as suggested by Alexei) and then
   everything just worked within test_progs; I didn't bother to check why it
   was failing with bigger max_entries;
v1->v2:
 - pretty-print BTF tests were renamed test_btf -> test_btf_pprint, which
   allowed GIT to detect that majority of  test_btf code was moved into
   prog_tests/btf.c; so diff is much-much smaller;

 tools/testing/selftests/bpf/.gitignore        |   1 -
 tools/testing/selftests/bpf/Makefile          |   2 +-
 .../bpf/{test_btf.c => prog_tests/btf.c}      | 410 ++++--------------
 3 files changed, 78 insertions(+), 335 deletions(-)
 rename tools/testing/selftests/bpf/{test_btf.c => prog_tests/btf.c} (96%)

tools/testing/selftests/bpf/.gitignore

@@ -15,7 +15,6 @@ test_sock
 test_sock_addr
 test_sock_fields
 urandom_read
-test_btf
 test_sockmap
 test_lirc_mode2_user
 get_cgroup_id_user

tools/testing/selftests/bpf/Makefile

@@ -33,7 +33,7 @@ LDLIBS += -lcap -lelf -lz -lrt -lpthread
 # Order correspond to 'make run_tests' order
 TEST_GEN_PROGS = test_verifier test_tag test_maps test_lru_map test_lpm_map test_progs \
 	test_verifier_log test_dev_cgroup test_tcpbpf_user \
-	test_sock test_btf test_sockmap get_cgroup_id_user test_socket_cookie \
+	test_sock test_sockmap get_cgroup_id_user test_socket_cookie \
 	test_cgroup_storage \
 	test_netcnt test_tcpnotify_user test_sock_fields test_sysctl \
 	test_progs-no_alu32 \