[2.0] Split the diffviewer unit tests into multiple files.

Review Request #520 — Created March 29, 2017 and updated — Latest diff uploaded

Review Board
The diff viewer unit tests were fairly extensive and were getting harder
to work with and expand. Merges were also getting more difficult, and
these were going to get worse with some changes that are in progress.

To make the unit tests easier to manage, they've been split up into
individual files based on area of responsibility. The code for the unit
tests have not changed, meaning we have some less-than-ideal imports,
docstrings, etc. To help faciliate merging this code (as there are 2.0
and 2.5 versions of this change), any and all cleanup will be done as a
separate change once the 2.0 and 2.5 versions have landed.

Testing Done:
Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/8826/