Add tests, remove `main` in `LinearSearchThread`

[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 59.06%. Comparing base (b54cc21) to head (e8f3246). Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5671      +/-   ##
============================================
+ Coverage     58.59%   59.06%   +0.46%     
- Complexity     3749     3770      +21     
============================================
  Files           554      556       +2     
  Lines         15922    15924       +2     
  Branches       3030     3037       +7     
============================================
+ Hits           9330     9406      +76     
+ Misses         6195     6115      -80     
- Partials        397      403       +6     

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