Update tests to improve coverage of some modules

[dpvc]

Thus PR adds some tests to get more coverage of some of the recent changes.

It also adjusts BaseItems.ts to remove some unneeded ?, as the TeXAtom gets an inferred mrow child automatically when created, so (unless someone deletes them explicitly), they will always be available.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.77%. Comparing base (27c47f3) to head (0a0f8e5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1378      +/-   ##
===========================================
+ Coverage    86.66%   86.77%   +0.10%     
===========================================
  Files          338      338              
  Lines        84384    84384              
  Branches      4779     4792      +13     
===========================================
+ Hits         73131    73221      +90     
+ Misses       11253    11140     -113     
- Partials         0       23      +23     

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