jseutter reproduced it after 112 and 49 runs on Snow Leopard using "regrtest.py -F -v --timeout=60 test_threadsignals" command:
-----------
[ 49] test_threadsignals
test_interrupted_timed_acquire (test.test_threadsignals.ThreadSignals) ... ok
test_lock_acquire_interruption (test.test_threadsignals.ThreadSignals) ... ok
test_lock_acquire_retries_on_intr (test.test_threadsignals.ThreadSignals) ... ok
test_rlock_acquire_interruption (test.test_threadsignals.ThreadSignals) ... ok
test_rlock_acquire_retries_on_intr (test.test_threadsignals.ThreadSignals) ... ok
test_signals (test.test_threadsignals.ThreadSignals) ... test_signals: acquire lock (thread 140735082572960)
test_signals: wait lock (thread 140735082572960)
Thread 0x00007fff709aaca0:
  File "/Users/jseutter/code/python/cpython/Lib/test/test_threadsignals.py", line 55 in test_signals
  File "/Users/jseutter/code/python/cpython/Lib/unittest/case.py", line 387 in _executeTestPart
  File "/Users/jseutter/code/python/cpython/Lib/unittest/case.py", line 442 in run
  File "/Users/jseutter/code/python/cpython/Lib/unittest/case.py", line 494 in __call__
  File "/Users/jseutter/code/python/cpython/Lib/unittest/suite.py", line 105 in run
  File "/Users/jseutter/code/python/cpython/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/jseutter/code/python/cpython/Lib/unittest/suite.py", line 105 in run
  File "/Users/jseutter/code/python/cpython/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/jseutter/code/python/cpython/Lib/unittest/runner.py", line 168 in run
  File "/Users/jseutter/code/python/cpython/Lib/test/support.py", line 1166 in _run_suite
  File "/Users/jseutter/code/python/cpython/Lib/test/support.py", line 1192 in run_unittest
  File "/Users/jseutter/code/python/cpython/Lib/test/test_threadsignals.py", line 221 in test_main
  File "Lib/test/regrtest.py", line 1032 in runtest_inner
  File "Lib/test/regrtest.py", line 826 in runtest
  File "Lib/test/regrtest.py", line 650 in main
  File "Lib/test/regrtest.py", line 1607 in <module>
-----------

So for an unknown reason, send_signals() thread was not created or doesn't start.

Example of the output of a success:
----------
test_signals: acquire lock (thread 140735082572960)
test_signals: wait lock (thread 140735082572960)
send_signals: enter (thread 4326952960)
send_signals: raise SIGUSR1
send_signals: raise SIGUSR2
send_signals: release signalled_all
send_signals: exit (thread 4326952960)
test_signals: lock acquired
----------