bpf, selftests: Add various BPF tc link selftests

[ Commit msg tbd ]

todo:
- selftest with old/legacy tc api
- selftest with chaining & termination

  # ./vmtest.sh -- ./test_progs -t tc_link
  [...]
  [    1.486605] clocksource: Switched to clocksource tsc
  ./test_progs -t tc_link
  [    1.542394] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.543803] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  torvalds#179     tc_link_base:OK
  torvalds#180     tc_link_mix:OK
  torvalds#181     tc_link_opts:OK
  torvalds#182     tc_link_run:OK
  Summary: 4/0 PASSED, 0 SKIPPED, 0 FAILED
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>