Tests to ensure custom args get passed into the experimental debugger

[DonJayamanne]

Fixes #1280

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

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The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1581   +/-   ##
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  Coverage          ?   71.44%           
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  Files             ?      273           
  Lines             ?    12700           
  Branches          ?     2282           
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  Hits              ?     9073           
  Misses            ?     3492           
  Partials          ?      135

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