Fix PayloadFormat, MultilineMultipleCueTextTag.

The test assumed whitespace between tags was skipped. Now
that we no longer to that, per spec, the tests must be
updated to expect intermediate text nodes.

This gets plformat_unittest passing again so it can be
re-enabled.
Merge pull request #322 from rillian/restore-plformat

Fix PayloadFormat, MultilineMultipleCueTextTag.