Improve the testing of test scripts

[vxgmichel]

... and prevent appveyor from getting stuck.

[codecov]

Codecov Report

Merging #906 into master will increase coverage by 12.74%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #906       +/-   ##
==========================================
+ Coverage   68.26%     81%   +12.74%     
==========================================
  Files         219     261       +42     
  Lines       16554   21337     +4783     
==========================================
+ Hits        11300   17285     +5985     
+ Misses       5254    4052     -1202

Impacted Files	Coverage Δ
parsec/core/gui/ui/claim_device_widget.py	100% <0%> (ø)
parsec/core/gui/ui/new_version_available.py	7.69% <0%> (ø)
parsec/core/gui/ui/mount_widget.py	100% <0%> (ø)
parsec/core/gui/ui/notification_widget.py	100% <0%> (ø)
parsec/core/gui/ui/notification_center_widget.py	100% <0%> (ø)
parsec/core/gui/ui/menu_widget.py	100% <0%> (ø)
parsec/core/gui/ui/about_widget.py	100% <0%> (ø)
parsec/core/gui/ui/login_widget.py	100% <0%> (ø)
parsec/core/gui/ui/loading_dialog.py	100% <0%> (ø)
parsec/core/gui/ui/devices_widget.py	100% <0%> (ø)
... and 85 more

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