Merge branch 'split-test_progs'

Stanislav Fomichev says:

====================
Recently we had linux-next bpf/bpf-next conflict when we added new
functionality to the test_progs.c at the same location. Let's split
test_progs.c the same way we recently split test_verifier.c.

I follow the same patten we did in commit 2dfb40121ee8 ("selftests: bpf:
prepare for break up of verifier tests") for verifier: create
scaffolding to support dedicated files and slowly move the tests into
separate files.

The first patch adds scaffolding, subsequent patches move progs into
separate files.

In theory, many of the standalone tests can be migrated to this new
framework as well. They get the benefit of common CHECK macro and
bpf_find_map function which a lot of standalone tests need to redefine.

v3 changes:
* respin on top of commit ebace0e981b2 ("selftests/bpf: use
  __bpf_constant_htons in test_prog.c for flow dissector")
* put bpf_rlimit.h into test_progs.c instead of test_progs.h

v2 changes:
* added cover letter, added more description about file structure
====================

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