selftests/bpf: test_progs: convert test_sockopt_inherit

Move the files, adjust includes, remove entry from Makefile & .gitignore

I also added pthread_cond_wait for the server thread startup. We don't
want to connect to the server that's not yet up (for some reason
this existing race is now more prominent with test_progs).

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>