perf test: Dump the stack when test segfaults when in verbose mode

E.g.:

  # perf test 26
  26: Test mmap thread lookup                                  : FAILED!
  # perf test -v 26
  26: Test mmap thread lookup                                  :
  --- start ---
  test child forked, pid 9269
  tid = 9269, map = 0x7ff99ff0c000
  tid = 9270, map = 0x7ff99ff0b000
  tid = 9271, map = 0x7ff99ff0a000
  tid = 9272, map = 0x7ff99ff09000
  perf: Segmentation fault
  Obtained 13 stack frames.
  perf(sighandler_dump_stack+0x41) [0x4e3541]
  /lib64/libc.so.6(+0x34960) [0x7ff99d5f6960]
  perf(thread__put+0x5b) [0x4c6f6b]
  perf(machine__process_event+0x14e) [0x4bd37e]
  perf(perf_event__synthesize_threads+0x3aa) [0x48678a]
  perf(test__mmap_thread_lookup+0x20a) [0x474e0a]
  perf() [0x460d56]
  perf(cmd_test+0x589) [0x461319]
  perf() [0x47c641]
  perf(main+0x617) [0x422317]
  /lib64/libc.so.6(__libc_start_main+0xf0) [0x7ff99d5e1fe0]
  perf() [0x422429]
  [(nil)]
  test child interrupted
  ---- end ----
  Test mmap thread lookup: FAILED!
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-sypazzsl4ptctrmlyi2zcmaj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>