Merge branch 'revamp-test_progs'

Merge branch 'revamp-test_progs'

Andrii Nakryiko says:

====================
This patch set makes a number of changes to test_progs selftest, which is
a collection of many other tests (and sometimes sub-tests as well), to provide
better testing experience and allow to start convering many individual test
programs under selftests/bpf into a single and convenient test runner.

Patch #1 fixes issue with Makefile, which makes prog_tests/test.h compiled as
a C code. This fix allows to change how test.h is generated, providing ability
to have more control on what and how tests are run.

Patch #2 changes how test.h is auto-generated, which allows to have test
definitions, instead of just running test functions. This gives ability to do
more complicated test run policies.

Patch #3 adds `-t <test-name>` and `-n <test-num>` selectors to run only
subset of tests.

Patch #4 changes libbpf_set_print() to return previously set print callback,
allowing to temporarily replace current print callback and then set it back.
This is necessary for some tests that want more control over libbpf logging.

Patch #5 sets up and takes over libbpf logging from individual tests to
test_prog runner, adding -vv verbosity to capture debug output from libbpf.
This is useful when debugging failing tests.

Patch #6 furthers test output management and buffers it by default, emitting
log output only if test fails. This give succinct and clean default test
output. It's possible to bypass this behavior with -v flag, which will turn
off test output buffering.

Patch #7 adds support for sub-tests. It also enhances -t and -n selectors to
both support ability to specify sub-test selectors, as well as enhancing
number selector to accept sets of test, instead of just individual test
number.

Patch #8 converts bpf_verif_scale.c test to use sub-test APIs.

Patch #9 converts send_signal.c tests to use sub-test APIs.

v2->v3:
  - fix buffered output rare unitialized value bug (Alexei);
  - fix buffered output va_list reuse bug (Alexei);
  - fix buffered output truncation due to interleaving zero terminators;

v1->v2:
  - drop libbpf_swap_print, instead return previous function from
    libbpf_set_print (Stanislav);
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>