tools/bpf: add selftest in test_progs for bpf_send_signal() helper

The test covered both nmi and tracepoint perf events.
  $ ./test_progs
  ...
  test_send_signal_tracepoint:PASS:tracepoint 0 nsec
  ...
  test_send_signal_common:PASS:tracepoint 0 nsec
  ...
  test_send_signal_common:PASS:perf_event 0 nsec
  ...
  test_send_signal:OK

Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>