Run coverage in parallel mode

[benjaoming]

Summary

Been wondering a bit why coverage isn't showing up properly for kalite.cli so trying out this, even though I think CodeCov already knows that it's receiving separate reports from both CircleCI instances...

Supersedes #5443

[codecov]

Codecov Report

Merging #5445 into 0.17.x will increase coverage by 9.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           0.17.x    #5445      +/-   ##
==========================================
+ Coverage   51.94%   61.39%   +9.45%     
==========================================
  Files         143      118      -25     
  Lines        7466     6574     -892     
==========================================
+ Hits         3878     4036     +158     
+ Misses       3588     2538    -1050

Impacted Files	Coverage Δ
kalite/coachreports/api_urls.py	100% <0%> (ø)	⬆️
kalite/contentload/api_urls.py	100% <0%> (ø)	⬆️
kalite/updates/urls.py	100% <0%> (ø)	⬆️
kalite/inline/api_urls.py	100% <0%> (ø)	⬆️
kalite/distributed/forms.py	100% <0%> (ø)	⬆️
kalite/control_panel/urls.py	100% <0%> (ø)	⬆️
kalite/inline/narratives.py	100% <0%> (ø)	⬆️
...lite/control_panel/features/steps/control_panel.py
kalite/project/settings/base.py
kalite/distributed/features/steps/generic.py
... and 58 more

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