test: redo undo with text formatting

[MayurSMahajan]

Related to Issue #16
Added new tests to cover the following:

• using redo before undo, does not affect the document
• formatting text bold and then using undo and redo
• formatting text italics and then using undo and redo
• formatting text with underline and then using undo and redo

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #38 (49466ba) into main (06240a0) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   80.42%   80.49%   +0.07%     
==========================================
  Files         123      123              
  Lines        7094     7094              
==========================================
+ Hits         5705     5710       +5     
+ Misses       1389     1384       -5     

see 2 files with indirect coverage changes

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