Improve test-coverage for TimedText::Cue

There are a couple of things that are not being tested here, based on the lcov reports.

Currently:
- Setting WebVTT attributes on the EmptyCue (Cue::type() == Cue::EmptyCue) is not tried at all.
- setId(), setText(), setStartTime() and setEndTime() are not tested at all, as of yet
- applySettings() are not being tested yet
- setVertical(enum) with invalid values is not tested yet
- setLine(string) with null or 0-length string is not tested yet

All of these should bring Cue.cpp to 100% test coverage, for now! Let us do it!