Fix plescape tests to expect correct value.

After webvtt_skipwhite() change some of the escape character
tests are expecting the incorrect value.
Fix AmpersandOnCurrLineWithSubclass.

- This test was using a .vtt file that was already tested.
- Changed to use ampersand-outside-tag-with-subclass.vtt which hasn't
been used and is a good match.
Fix AmpersandOnCurrLineWithClass.

- This unit test used a .vtt file that was already in use.
- Renamed and set to use a .vtt file that is not in use yes
and for which it was probably designated.
- Removed plescapecharacter_unittest from XFAIL because it is
now passing.