selftests: Fix test group SKIPPED result

When showing the result of a test group, if one
of the subtests was skipped, while still having
passing subtests, the group result was marked as
SKIP. E.g.:

223/1   usdt/basic:SKIP
223/2   usdt/multispec:OK
223/3   usdt/urand_auto_attach:OK
223/4   usdt/urand_pid_attach:OK
223     usdt:SKIP

The test result of usdt in the example above
should be OK instead of SKIP, because the test
group did have passing tests and it would be
considered in "normal" state.

With this change, only if all of the subtests
were skipped, the group test is marked as SKIP.
When only some of the subtests are skipped, a
more detailed result is given, stating how
many of the subtests were skipped. E.g:

223/1   usdt/basic:SKIP
223/2   usdt/multispec:OK
223/3   usdt/urand_auto_attach:OK
223/4   usdt/urand_pid_attach:OK
223     usdt:OK (SKIP: 1/4)

Signed-off-by: Domenico Cerasuolo <dceras@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221109184039.3514033-1-cerasuolodomenico@gmail.com