selftests/bpf: Add per worker cgroup suffix

This patch make each worker use a unique cgroup base directory, thus
allowing tests that uses cgroups to run concurrently.

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211006185619.364369-5-fallentree@fb.com