Changed property name used to capture the trigger source of Unit Tests.

[DonJayamanne]

Fixes #1213

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 #1214 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
+ Coverage   71.91%   71.98%   +0.06%     
==========================================
  Files         260      260              
  Lines       11933    11933              
  Branches     2121     2121              
==========================================
+ Hits         8582     8590       +8     
+ Misses       3221     3217       -4     
+ Partials      130      126       -4

Impacted Files	Coverage Δ
...lient/unittests/common/managers/baseTestManager.ts	77.5% <ø> (ø)	⬆️
src/client/debugger/Main.ts	50.61% <0%> (+0.49%)	⬆️
...rc/client/debugger/PythonProcessCallbackHandler.ts	52.96% <0%> (+0.65%)	⬆️
src/client/debugger/PythonProcess.ts	47.08% <0%> (+1.66%)	⬆️

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