test_timelyProcessExited is fragile and may not catch actual problems

[itamar]

JP pointed out that test_timelyProcessExited in twisted.internet.test_process may fail without us noticing:

The problem is that the timeout support in runReactor is based on the IReactorTime implementation of the reactor it is running. It's quite likely that if the reactor is broken, instead of failing:

1. the child will exit
2. the reactor will not process the signal right away
3. the timeout will elapse
4. the reactor will wake up
5. the signal will be processed, calling processEnded
6. reactor.stop() will be called
7. runReactor will notice the reactor was stopped and not raise a timeout exception
8. the test will pass

[itamarst]

(In [34411]) Branching to 'process-test-5680'

[itamar]

Ready for review; also covers #5679. I scheduled but haven't yet checked a buildbot run:

 http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/process-test-5680

[therve]

Looks good. I would add a parameter to assertTrue with the value of exist - start so that we can debug it in case of problems. Please merge after that.