The LoopingCall tests in twisted.test.test_task encode implementation details of Clock

[exarkun]

As reported in #5962, Clock and the real reactor differ in their handling of callLater(0, ...). Unfortunately the LoopingCall tests in twisted.test.test_task encode a dependency on the Clock behavior so they only demonstrate that LoopingCall has some behavior when used with a broken reactor.

I have a fix for #5962 but I don't want to complicate its PR with a lot of subtle changes to test suites for other units. So, in preparation for landing that work, let's fix these tests to have the minimum possible dependency on Clock's quirks.