Throw error when mocha parallel is set to true

[cgewecke]

Per discussion in 691 comment

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (658dc37) 96.48% compared to head (cfee11d) 96.49%.

❗ Current head cfee11d differs from pull request most recent head 9397d1a. Consider uploading reports for the commit 9397d1a to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #833   +/-   ##
=======================================
  Coverage   96.48%   96.49%           
=======================================
  Files          19       19           
  Lines        1054     1057    +3     
=======================================
+ Hits         1017     1020    +3     
  Misses         37       37           

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