test_faulthandler: fix regex on the check_dump_traceback_threads() traceback

test_faulthandler: fix regex on the check_dump_traceback_threads() traceback

The traceback may contain "_is_owned":

Thread 0x40962b90:
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/threading.py", line 220 in _is_owned
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/threading.py", line 227 in wait
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/threading.py", line 421 in wait
  File "<string>", line 23 in run
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/threading.py", line 735 in _bootstrap_inner
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/threading.py", line 708 in _bootstrap

Current thread XXX:
  File "<string>", line 10 in dump
  File "<string>", line 28 in <module>