Enable multi-threaded tests for experimental debugger

[DonJayamanne]

Fixes #1250

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]

Simple...

[codecov]

Codecov Report

Merging #1935 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1935      +/-   ##
==========================================
- Coverage   74.35%   74.28%   -0.07%     
==========================================
  Files         283      283              
  Lines       13287    13287              
  Branches     2387     2387              
==========================================
- Hits         9879     9870       -9     
- Misses       3273     3278       +5     
- Partials      135      139       +4

Impacted Files	Coverage Δ
src/client/debugger/PythonProcess.ts	45.83% <0%> (-2.09%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	52.96% <0%> (-1.32%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3a614bc...86afee6. Read the comment docs.