Fix cmap changing in `ttFont` under test

[RickyDaMa]

Description

Relates to race condition issues with --auto-jobs

Deep copies the ttFont in com.google.fonts/check/tabular_kerning so the cmap modifications don't trip up other tests. In our case, I saw com.google.fonts/check/soft_dotted crash because the cmap was changed while it was being iterated through

For the nuance between copy and deepcopy, see: https://docs.python.org/3/library/copy.html

Props to @Hoolean for correctly guessing and then spotting this issue from an awful stack trace in a one-off error

Checklist

• [x] update CHANGELOG.md
• [x] wait for the tests to pass
• [ ] request a review

[felipesanches]

This will resume at PR #4709