Great! The timeout works:
-------------------------------------
...
[ 25/354] test_threadsignals
Thread 0xa000ed88:
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/test_threadsignals.py", line 46 in test_signals
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/case.py", line 387 in _executeTestPart
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/case.py", line 442 in run
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/case.py", line 494 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/support.py", line 1078 in run
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/support.py", line 1166 in _run_suite
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/support.py", line 1192 in run_unittest
  File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/test_threadsignals.py", line 210 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>
make: *** [buildbottest] Error 1
program finished with exit code 2
elapsedTime=1103.489396
-------------------------------------
http://www.python.org/dev/buildbot/all/builders/PPC%20Tiger%203.x/builds/1685/steps/test/logs/stdio

Compare it without the same failure without the timeout:
-------------------------------------
...
[106/354] test_threadsignals

command timed out: 3600 seconds without output, killing pid 1745
process killed by signal 9
program finished with exit code -1
elapsedTime=4785.840329
-------------------------------------
http://www.python.org/dev/buildbot/all/builders/PPC%20Tiger%203.x/builds/1684/steps/test/logs/stdio

Thanks to this timeout, the build stops after 18 minutes instead of 80 minutes.... 18 minutes including the timeout of 15 minutes :-)

test_threadsignals hangs at:
----------------
class ThreadSignals(unittest.TestCase):
    def test_signals(self):
        signalled_all.acquire()
        self.spawnSignallingThread()
        signalled_all.acquire() <~~~~ here
        ...
----------------

self.spawnSignallingThread() calls:
----------------
def send_signals():
    os.kill(process_pid, signal.SIGUSR1)
    os.kill(process_pid, signal.SIGUSR2)
    signalled_all.release()
----------------