Port `twisted.internet.test.reactormixins` to Python 3

[exarkun]

This module is the basis of a decent sized chunk of the test suite for the reactor implementations. Porting it to Python 3 is a necessary precondition to being able to run that part of the test suite on Python 3, and therefore a precondition for being able to port the reactors to Python 3.

[exarkun]

(In [35593]) Branching to 'reactormixins-python3-5967'

[exarkun]

Moved some code around. Adjusted some imports. Removed use of assertFailure from test_unix.py, to work with change from TestCase to SynchronousTestCase in reactormixins.py.  Build results

[exarkun]

(In [35606]) Switch another instance of assertFailure to manual Failure checking

refs #5967

[itamar]

1. filter() is lazy in Python 3, it seems.
2. Given the above wouldn't have been caught by anything, we should open a ticket for writing tests for ReactorBuilder. Especially since I want to expose some of this infrastructure, to allow users to write tests that have a reactor per-test (unless we have a full fake reactor that'll be ready soon).
3. Given we've both implemented variations, possibly SyncTestCase should have a sync version of assertFailure? Or maybe the use cases are sufficiently diverse that there's no point.
4. Add future imports.

Please fix, and merge.

[exarkun]

(In [35611]) Avoid lazy filter. Also fix __all__ definition.

refs #5967

[exarkun]

(In [35614]) Merge reactormixins-python3-5967

Author: exarkun Reviewer: itamar Fixes: #5967

Port twisted.internet.test.reactormixins to Python 3, in anticipation of porting some tests based on it to Python 3.