selftests/bpf: Add two selftests for mprog API based cgroup progs

Two tests are added:
  - cgroup_mprog_opts, which mimics tc_opts.c ([1]). Both prog and link
    attach are tested. Some negative tests are also included.
  - cgroup_mprog_ordering, which actually runs the program with some mprog
    API flags.

  [1] https://github.com/torvalds/linux/blob/master/tools/testing/selftests/bpf/prog_tests/tc_opts.c

Signed-off-by: Yonghong Song <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Yonghong Song