Text layout testing

[kpreid]

Followup to #775 which introduced a new text renderer, but this was already true:

The text layout code should be more thoroughly tested. In particular, I think there should be unit tests of layout by itself, not involving any rendered image comparison but checking the relationship of glyph bounding boxes to the requested positioning.

[kpreid]

Added some of these tests in f89da16 and a99d652. However, it turns out there already were some — specifically for the X axis only — in https://github.com/kpreid/all-is-cubes/blob/aee72bf4f6f4e12aa178c0fcdc0dd45fc901688c/test-aic/tests/test-aic/block_text.rs. Not sure which approach is best.