Use thenResolve on mocks instead of returning a promise

[Nateowami]

Converts someMethod.thenReturn(Promise.resolve(resolution)) => someMethod.thenResolve(resolution)

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.80%. Comparing base (ed7cdd6) to head (aa68fb6).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3631   +/-   ##
=======================================
  Coverage   82.80%   82.80%           
=======================================
  Files         610      610           
  Lines       37446    37446           
  Branches     6139     6139           
=======================================
  Hits        31009    31009           
  Misses       5500     5500           
  Partials      937      937           

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[pmachapman]

@pmachapman reviewed 13 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Nateowami).