Fix failing Prospector unit tests and add more tests for linters

[DonJayamanne]

Fixes #1836

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1837 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1837      +/-   ##
==========================================
- Coverage    74.3%   74.27%   -0.04%     
==========================================
  Files         282      282              
  Lines       13191    13191              
  Branches     2371     2371              
==========================================
- Hits         9801     9797       -4     
- Misses       3259     3262       +3     
- Partials      131      132       +1

Impacted Files	Coverage Δ
...reter/locators/services/cacheableLocatorService.ts	91.83% <0%> (-4.09%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	52.96% <0%> (-1.32%)	⬇️
src/client/debugger/Main.ts	52.34% <0%> (+0.49%)	⬆️

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