selftests/bpf: Switch test_tcpbpf_user to cgroup_helpers

Switch to cgroup_helpers to simplify the code and fix cgroup cleanup:
before cgroup was not cleaned up after the test.

It also removes SYSTEM macro, that only printed error, but didn't
terminate the test.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>