selftests/bpf: Add support for marking sub-tests as skipped

Previously skipped sub-tests would be counted as passing with ":OK" appened
in the log. Change that to be accounted as ":SKIP".

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201203204634.1325171-8-andrii@kernel.org