Merge branch 'PROG_TEST_RUN support for sk_lookup programs'

Lorenz Bauer says:

====================

We don't have PROG_TEST_RUN support for sk_lookup programs at the
moment. So far this hasn't been a problem, since we can run our
tests in a separate network namespace. For benchmarking it's nice
to have PROG_TEST_RUN, so I've gone and implemented it.

Based on discussion on the v1 I've dropped support for testing multiple
programs at once.

Changes since v3:
- Use bpf_test_timer prefix (Andrii)

Changes since v2:
- Fix test_verifier failure (Alexei)

Changes since v1:
- Add sparse annotations to the t_* functions
- Add appropriate type casts in bpf_prog_test_run_sk_lookup
- Drop running multiple programs
====================

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