Unit testing paid off already, because I discovered two bugs while writing the tests.
What's incomplete is coverage and further testing of the scrollbars themselves, in particular a simulation of dragability.
Code Coverage Result: 81.29%
Missing Code Coverage: gimmicky.FlxScrollbar [61.45%]
Because FlxScrollbar has to update FlxScrollableArea as it goes, I've left it loosely coupled, but this means that to test the former to full coverage, I will need to mock and stub the latter. Stay tuned for adventures with Mockatoo! (Yeah, because blogs are tuned into. Don't forget to jump up and down and hold your arms at funny angles to improve your reception.)