stopServer() never resolves with yarn

[miguelcobain]

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Spent the last two hours trying to make tests pass on travis with ember-cli-addon-tests addon.

Turned out that yarn test failed, but npm test passed. 🤦‍♂️

With yarn the app.stopServer() promise never resolves. Something in yarn is not letting us kill child processes (which is what stopServer does).

[xg-wang]

I have this issue on my mac after yarn 1.10.1 -> 1.12.3 and RHE7 1.9.4 -> 1.12.3