Investigate why CI got broken on master but PR passed

[gaearon]

See this PR: #9550.

It passed CI on PR, but when merged to master, it had a test failure (which I later fixed in #9854).

I wonder if we're missing some check in PRs.

[trueadm]

This puzzled me. I didn't understand how the test failed for us now when on the CI run, it ran the test and showed it as passing:

Then I remembered that we made a change to make tests properly run on CI a while back. So I think this is related to the CI change we made to the Jest task runner not turning an exit:

https://github.com/facebook/react/blob/7052d7c71071710ad396cf52d3ded9898c3d057d/scripts/tasks/jest.js

[gaearon]

Thanks for investigating!
To clarify: the issue is that the PR's merge base did not include the fix from #9663.

We might have some more PRs like this, so need to be mindful (and ideally rebase old PRs before merging).