testing: improve test coverage SortedLinkedListTest

[alxkm]

Added additional test cases to SortedLinkedListTest to improve coverage.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.65%. Comparing base (054002a) to head (e95087f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6388      +/-   ##
============================================
- Coverage     74.65%   74.65%   -0.01%     
- Complexity     5458     5459       +1     
============================================
  Files           682      682              
  Lines         19010    19010              
  Branches       3670     3670              
============================================
- Hits          14192    14191       -1     
- Misses         4258     4259       +1     
  Partials        560      560              

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