selftests/bpf: Adding pid filtering for atomics test

This make atomics test able to run in parallel with other tests.

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211006185619.364369-11-fallentree@fb.com