Move escape-char tests escape-char unit test file.

Some of the escape character tests were incorrectly placed into the payload format
test file.
Fix PayloadEscappeCharacter.AmpersandWithinTag

- The .vtt test file's ampersand escape character is formmated correctly
so the unit test should expect '&' instead of '&amp;'.
- The italic node's index in the .vtt file is head[ 0 ] not head.