Fix unit tests for increment/decrement font size

This is for #3794. A recent change to try fix #3478 exposed the inaccuracy of these tests. Specifically, changing the ratio of line-height to font-size from 1.25 to 1.3 broke these tests. Note that #3478 is still not fixed.

Making these tests perfectly accurate would require either some new function(s) in Editor, or to largely duplicate the code in ViewCommandHandlers._adjustFontSize(). If that's desired, then these tests should be disabled for now.

Otherwise, I found a possibly reasonable way to fix these unit tests. I also added comments to the tests to explain their shortcomings.