Move almost 200 tests from test_btf into test_progs framework to be exercised

regularly. Pretty-printing tests were left alone in test_btf because they are
very slow and were not even executed by default with test_btf. Also, they seem
to break when moved under test_progs and I didn't want to spend more time on
figuring out why they broke, given we don't want to execute them all the time
anyway.

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 close to the original as possible. `test_progs -v` will
activate "always_log" flag to emit BTF validation log.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 tools/testing/selftests/bpf/prog_tests/btf.c | 6124 ++++++++++++++
 tools/testing/selftests/bpf/test_btf.c       | 7442 ++----------------
 2 files changed, 6796 insertions(+), 6770 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/btf.c

Author: anakryiko