adding tests for string append

[jaredwray]

Please check if the PR fulfills these requirements

• Followed the Contributing guidelines.
• Tests for the changes have been added (for bug fixes/features) without decreasing code coverage.
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
adding tests for string append

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.09% 🎉

Comparison is base (7fe6a96) 96.49% compared to head (fcfde4c) 96.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   96.49%   96.58%   +0.09%     
==========================================
  Files          26       26              
  Lines        4425     4425              
==========================================
+ Hits         4270     4274       +4     
+ Misses        155      151       -4     

see 1 file with indirect coverage changes

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