selftests/bpf: Add parallel support for classid

selftests/bpf: Add parallel support for classid

Include the current pid in the classid cgroup path. This way, different
testers relying on classid-based configurations will have distinct classid
cgroup directories, enabling them to run concurrently. Additionally, we
leverage the current pid as the classid, ensuring unique identification.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20231111090034.4248-4-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>