selftests/bpf: Add ASSERT_STRNEQ() variant for test_progs

It will support strncmp()-style string comparisons.

Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1626362126-27775-3-git-send-email-alan.maguire@oracle.com