Fix flask app variable in debug configuration and flask unit tests

[DonJayamanne]

Fixes #1634
Fixes #1640

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)

[d3r3kk]

Fine.

[d3r3kk]

spelling: experimental

[codecov]

Codecov Report

Merging #1641 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1641      +/-   ##
==========================================
- Coverage   71.62%   71.53%   -0.09%     
==========================================
  Files         277      277              
  Lines       12816    12816              
  Branches     2299     2299              
==========================================
- Hits         9179     9168      -11     
- Misses       3502     3508       +6     
- Partials      135      140       +5

Impacted Files	Coverage Δ
src/client/debugger/PythonProcess.ts	45.83% <0%> (-5.42%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	54.27% <0%> (+0.65%)	⬆️

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