Show Flow errors in yarn test (as a Jest reporter)

[kumar303]

This adds a custom Jest reporter that shows Flow errors in our test output.

In practical usage, I don't think there will be any impact to speed. There might be a couple seconds of delay when the server starts for the first time but that happens in parallel to the tests anyway.

This fixes mozilla/addons#12498 by removing the need to run yarn flow:dev. It even implements part of mozilla/addons#10342 🍰

I've actually wanted to see Flow errors in test output for a while but for some reason it never struck me to create a reporter! It also seems like a decent workaround to jestjs/jest#7124

Some examples:

Flow failures

No Flow errors

[kumar303]

@willdurand I'm curious to see how this works for you since you don't use watch mode.

[codecov-io]

Codecov Report

Merging #6617 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    mozilla/addons-frontend#6617   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files         239      239           
  Lines        6519     6519           
  Branches     1248     1248           
=======================================
  Hits         6378     6378           
  Misses        126      126           
  Partials       15       15

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[willdurand]

That works for me, thanks! r+