test_itimer_virtual assumes that a process must get 0.3s of virtual
time within 5s of real time. This is not true:

I can easily make the test fail even on a fast machine by doing
as root (do it n times for n cores):

nice -n -19 sh -c 'echo "1234^123456789" | bc'


======================================================================
FAIL: test_itimer_prof (__main__.ItimerTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_signal.py", line 401, in test_itimer_prof
    self.fail('timeout waiting for sig_prof signal')
AssertionError: timeout waiting for sig_prof signal

======================================================================
FAIL: test_itimer_virtual (__main__.ItimerTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_signal.py", line 379, in test_itimer_virtual
    (signal.getitimer(self.itimer),))
AssertionError: timeout waiting for sig_vtalrm signal; signal.getitimer(self.itimer) gives: (0.200012, 0.200012)