selftests/bpf: Add selftests for netkit

Add a bigger batch of test coverage to assert correct operation of
netkit devices and their BPF program management:

  # ./test_progs -t tc_netkit
  [...]
  [    1.166267] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.166831] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  [    1.270957] tsc: Refined TSC clocksource calibration: 3407.988 MHz
  [    1.272579] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x311fc932722, max_idle_ns: 440795381586 ns
  [    1.275336] clocksource: Switched to clocksource tsc
  #257     tc_netkit_basic:OK
  #258     tc_netkit_device:OK
  #259     tc_netkit_multi_links:OK
  #260     tc_netkit_multi_opts:OK
  #261     tc_netkit_neigh_links:OK
  Summary: 5/0 PASSED, 0 SKIPPED, 0 FAILED
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20231024214904.29825-8-daniel@iogearbox.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>