[Type tester] language exemplar chars for text subsets

The Type Tester / Specimen does not use a subset that is relevant for the Continent/Language viewed.

For example:
- Go to https://fonts.google.com/specimen/Questrial/tester
- Select All Continents and Yoruba
will show the following (fallback font and tofu):

**Expected behavior**
The Type Tester / Specimen should either use the subset that covers the language (many language are not fully covered by several predefined subsets) or use the language exemplar chars for text-based subset.