Split the unit/font tests into per-browser jobs

[timvandermeij]

This commit mirrors the approach from e656b83 to the other workflows that run multiple OS/browser combinations. This approach has multiple advantages:

• it improves performance because each job is run in its own environment so we don't have two browsers competing for resources in the same environment anymore;
• it improves monitoring because each job is shown separately, with its own runtime, in e.g. the pull request checks and actions overviews, which makes it easier to spot bottlenecks that are specific to a certain OS/browser combination and enable follow-up optimizations.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.77%. Comparing base (e66fdfb) to head (485dcb7).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21183      +/-   ##
==========================================
+ Coverage   63.32%   63.77%   +0.44%     
==========================================
  Files         206      213       +7     
  Lines      134464   134573     +109     
==========================================
+ Hits        85152    85825     +673     
+ Misses      49312    48748     -564     

Flag	Coverage Δ
fonttest	8.66% <ø> (?)
unittest	38.86% <ø> (?)
unittestcli	63.32% <ø> (ø)

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[calixteman]

LGTM. THank you.