Add additional interesting test ttx files for glyph loading

Per https://github.com/googlefonts/fontations/pull/191#pullrequestreview-1246917282, we want more ttx files for testing glyph loading with interesting cases. We'll also want some variable fonts to test when we get tuple variation store support in read-fonts. And of course, hinted fonts at some point.

Making this an issue to avoid blocking that PR on ttx curation.

Review the linked pull request review and the read-fonts testing setup to find where additional ttx fixtures belong. The issue does not name specific files or cases, so clarify which interesting glyph-loading cases are wanted; done means adding the agreed fixtures, with variable and hinted fonts deferred until the stated support exists.

Written by the indexing model from the issue text.