White box tests for twisted.internet.process

[Itamar Turner-Trauring]

It would be nice if in twisted.internet.test.test_process test_processExitedRaises could eventually stop poking the process module. I think this means we should have some direct unit tests for twisted.internet.process (perhaps in twisted/internet/test/test_posixprocess.py) for various exceptional cases of reapAllProcesses, registerReapProcessHandler, and unregisterReapProcessHandler. These can be as whitebox as necessary, since twisted.internet.process is just a big implementation detail. The other cases where an exception might be raised are interesting too, and we want to make sure they're also being handled correctly.

(Originally written by JP.)