We had a lot of old unit tests for the diff viewer that relied upon
locally-stored files. Over time, we moved away from this and moved
toward inline test data. This change completes the move in the diff
viewer test suite.

This has the advantage of making it clear what newline characters were
being used, or tab characters, or form feed characters. It also
prevents problems when dealing with newline normalization during
checkout on Windows.

It does increase the length of the test code, but it helps the
readability, since it's no longer required to figure out which test
data files are used. We also got to remove some old, now useless tests
that just tested basic parsing logic based on locally-generated diffs.

All unit tests pass.