Add tests, remove `main` in `LinearSearch`

[Hardvan]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] 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 :white_check_mark:

Project coverage is 61.77%. Comparing base (401d873) to head (5d745ac).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5670      +/-   ##
============================================
+ Coverage     61.69%   61.77%   +0.07%     
- Complexity     3955     3960       +5     
============================================
  Files           563      563              
  Lines         15960    15951       -9     
  Branches       3070     3070              
============================================
+ Hits           9847     9854       +7     
+ Misses         5702     5687      -15     
+ Partials        411      410       -1     

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