regrtest: use faulthandler to dump the tracebacks on SIGUSR1

Lib/test/regrtest.py

--- a/Lib/test/regrtest.py
+++ b/Lib/test/regrtest.py
@@ -160,21 +160,28 @@ To enable all resources except one, use 
 example, to run all the tests except for the gui tests, give the
 option '-uall,-gui'.
 """
+USAGE_SIGUSR1 = """
+You can send a SIGUSR1 signal to display the tracebacks of the current threads.
+Note that sending a signal may interrupt a system call in progress and thus
+interfere with the tests, so only do it if a test hangs and you want to know
+where.
+"""
 
 import builtins
+import errno
 import faulthandler
 import getopt
+import io
 import json
 import os
 import random
 import re
-import io
+import signal
 import sys
 import time
-import errno
 import traceback
+import unittest
 import warnings
-import unittest
 from inspect import isabstract
 import tempfile
 import platform
@@ -268,6 +275,8 @@ def main(tests=None, testdir=None, verbo
 
     # Display the Python traceback fatal errors (e.g. segfault)
     faulthandler.enable(all_threads=True)
+    if hasattr(signal, 'SIGUSR1'):
+        faulthandler.register(signal.SIGUSR1)
 
     if hasattr(faulthandler, 'dump_tracebacks_later'):
         timeout = 60*60
@@ -297,7 +306,10 @@ def main(tests=None, testdir=None, verbo
     start = None
     for o, a in opts:
         if o in ('-h', '--help'):
-            print(__doc__)
+            usage = __doc__
+            if hasattr(signal, 'SIGUSR1'):
+                usage += USAGE_SIGUSR1
+            print(usage)
             return
         elif o in ('-v', '--verbose'):
             verbose += 1