I've had a chance to do some testing and it _is_ related to the itimer
tests (in test_wait4).

This is with r66550:

$ ./python -E -tt ./Lib/test/regrtest.py -l -v test_wait4 test_signal
test_wait4
test_wait (test.test_wait4.Wait4Test) ... ok

----------------------------------------------------------------------
Ran 1 test in 5.007s

OK
test_signal
test_getsignal (test.test_signal.BasicSignalTests) ... ok
test_out_of_range_signal_number_raises_error
(test.test_signal.BasicSignalTests) ... ok
test_setting_signal_handler_to_none_raises_error
(test.test_signal.BasicSignalTests) ... ok
test_main (test.test_signal.InterProcessSignalTests) ... FAIL
test_wakeup_fd_during (test.test_signal.WakeupSignalTests) ... ok
test_wakeup_fd_early (test.test_signal.WakeupSignalTests) ... ok
test_siginterrupt_off (test.test_signal.SiginterruptTest) ... ok
test_siginterrupt_on (test.test_signal.SiginterruptTest) ... ok
test_without_siginterrupt (test.test_signal.SiginterruptTest) ... ok
test_itimer_exc (test.test_signal.ItimerTest) ... ok
test_itimer_prof (test.test_signal.ItimerTest) ...

Running only test_signal:
$ ./python -E -tt ./Lib/test/regrtest.py -l -v test_signal
test_signal
test_getsignal (test.test_signal.BasicSignalTests) ... ok
test_out_of_range_signal_number_raises_error
(test.test_signal.BasicSignalTests)
 ... ok
test_setting_signal_handler_to_none_raises_error
(test.test_signal.BasicSignalTe
sts) ... ok
test_main (test.test_signal.InterProcessSignalTests) ... ok
test_wakeup_fd_during (test.test_signal.WakeupSignalTests) ... ok
test_wakeup_fd_early (test.test_signal.WakeupSignalTests) ... ok
test_siginterrupt_off (test.test_signal.SiginterruptTest) ... ok
test_siginterrupt_on (test.test_signal.SiginterruptTest) ... ok
test_without_siginterrupt (test.test_signal.SiginterruptTest) ... ok
test_itimer_exc (test.test_signal.ItimerTest) ... ok
test_itimer_prof (test.test_signal.ItimerTest) ... ('SIGPROF handler
invoked', (
27, <frame object at 0x83c860c>))
ok
test_itimer_real (test.test_signal.ItimerTest) ...
call pause()...
('SIGALRM handler invoked', (14, <frame object at 0x83c8c0c>))
ok
test_itimer_virtual (test.test_signal.ItimerTest) ... ('SIGVTALRM
handler invoke
d', (26, <frame object at 0x83c920c>))
('SIGVTALRM handler invoked', (26, <frame object at 0x83c920c>))
('SIGVTALRM handler invoked', (26, <frame object at 0x83c920c>))
last SIGVTALRM handler call
('SIGVTALRM handler invoked', (26, <frame object at 0x83c920c>))
ok

----------------------------------------------------------------------
Ran 13 tests in 6.534s

OK
1 test OK.

Noting the interprocess signal test failure, I tried deactivating it and
the itimer tests still go off into the never never (using all available
CPU cycles too).