Add a setting for disabling auto-run test discover on save

[lingyv-li]

Fixes #1037

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

❗ No coverage uploaded for pull request base (master@98f8ad4). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #1687   +/-   ##
========================================
  Coverage          ?   71.5%           
========================================
  Files             ?     277           
  Lines             ?   13016           
  Branches          ?    2344           
========================================
  Hits              ?    9307           
  Misses            ?    3574           
  Partials          ?     135

Impacted Files	Coverage Δ
src/client/common/configSettings.ts	87.5% <ø> (ø)
src/client/common/types.ts	100% <ø> (ø)
src/client/unittests/main.ts	51.24% <100%> (ø)

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