We have a lot of unit tests that test the results of diff parsing, and

they all do it slightly differently. Some assume the number of resulting

ParsedDiffFile objects. Some fail to test certain state. Most just

haven't been updated as new attributes have been added. Most that are

more comprehensive ideally wouldn't be, since it's hard to tell what

state they're explicitly checking for and what are considered defaults.
There are some updates coming to diff parsing for DiffX and for some

in-progress SCM implementations, such as symlink target storage. To

prepare for that, this change goes through and formalizes how we test

these objects.
A new reviewboard.diffviewer.testing.mixins.DiffParserTestingMixin

class provides some assertions for these objects:

assert_parsed_diff_file()
`assert_parsed_diff_change()
assert_parsed_diff()

These take in attributes to check on the objects. Default values are

checked if not explicitly provided.
All relevant unit tests now use these methods, ensuring parsing changes

or state updates don't regress anything or get missed going forward.

All unit tests passed.