Before, the text chunker would assume that all text streams were in
MS, which is not a safe assumption to make.
This changes it to take the time scale from StreamInfo and work
natively with scaled time units.
This required updating the tests. While doing so the tests were
rewritten with the goal to make them easier to read.
Closes: #399
Change-Id: Ib792ad306f40d749763418cde645337913a6046b