Fix ciarrow_unittest +style issues

Wrapped lines in ciarrow_unittest.cpp to 80 columns
Make use of helper functions for error checks where possible
Expect rather than assert where possible
Safety checks to prevent buffer overflows/out of bounds access

No longer expecting WEBVTT_ID_TRUNCATED error -- However, we probably
do want some way of truncating cue IDs (should happen for IDs which
are really long! But I don't see an actual limit in the html5 draft on
ID lengths, so...)