Fix race condition in mainloop unittest

Currently, in testDeferredCancel if the self._CancelEvent are entered
more than 0.3 seconds after the _SendSig have been entered, the test
could fail. This is unlikely but may happen. To avoid it we use
enterabs, to use absolute times.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>