Add tests, remove `print` & `main` methods in `BoundaryFill.java`

[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 58.32%. Comparing base (0d68b65) to head (a5d2cf7).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5640      +/-   ##
============================================
+ Coverage     58.20%   58.32%   +0.11%     
- Complexity     3693     3699       +6     
============================================
  Files           549      549              
  Lines         15850    15841       -9     
  Branches       3015     3014       -1     
============================================
+ Hits           9226     9239      +13     
+ Misses         6232     6209      -23     
- Partials        392      393       +1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[Hardvan]

Thanks!