Warning in tests: "A worker process has failed to exit gracefully and has been force exited. This is likely caused by tests leaking due to improper teardown. Try running with --runInBand --detectOpenHandles to find leaks."

When running tests I get this warning sometimes:

> A worker process has failed to exit gracefully and has been force exited. This is likely caused by tests leaking due to improper teardown. Try running with --runInBand --detectOpenHandles to find leaks.

I tried to run the command that wouldn't find anything, so I believe this is intermittent.
Also I think this is the reason why appveyor's build is red sometimes, because I know it's more sensitive to these kind of errors.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-267)