Add tests, enhance class & function documentation for `KnightsTour.jav…

[Hardvan]

Also removed print and main methods.

• 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

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 54.80%. Comparing base (f34fe4d) to head (f85e3e0).

Files with missing lines	Patch %	Lines
...va/com/thealgorithms/backtracking/KnightsTour.java	60.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5591      +/-   ##
============================================
+ Coverage     54.56%   54.80%   +0.24%     
- Complexity     3413     3433      +20     
============================================
  Files           535      535              
  Lines         15383    15372      -11     
  Branches       2935     2932       -3     
============================================
+ Hits           8393     8425      +32     
+ Misses         6637     6587      -50     
- Partials        353      360       +7     

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