Replace Deprecated Test Case Assertion Methods in twisted.trial

[Julian]

See #5771.

Note that unlike the rest of the related tickets (#6220 through #6226 and #6230 through #6233) this one requires a bit more care to review (and more was taken to write hopefully). The tests that are specifically testing methods on trial's TestCase are left untouched, still testing the old methods.

Also, there are a few lines of change in twisted/trial/_synctest.py which change the implementation of things like assertSubstring to instead delegate to a non-deprecated method. If there were more I would have put it in its own ticket, but it's only 4-5 lines.