selftests/bpf: Replace CHECK with ASSERT_* in ns_current_pid_tgid test

Replace CHECK in selftest ns_current_pid_tgid with recommended ASSERT_* style.
I also shortened subtest name as the prefix of subtest name is covered
by the test name already.

This patch does fix a testing issue. Currently even if bss->user_{pid,tgid}
is not correct, the test still passed since the clone func returns 0.
I fixed it to return a non-zero value if bss->user_{pid,tgid} is incorrect.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240315184859.2975543-1-yonghong.song@linux.dev