title: Firebase emulators:exec --only firestore not executing the script given as parameter
body: <p>I used to run the following command to run some tests against a emulated firestore instance:</p>
  <p><code>firebase emulators:exec --only firestore 'npm test'</code></p>
  <p>Since my upgrade to <code>&quot;firebase-functions&quot;: &quot;^3.18.0&quot;</code> and `&quot;firebase-functions-test&quot;: &quot;^0.3.3&quot;' this doesn't execute my tests anymore. And after a minute gives me:
  Error: TIMEOUT: Port 8080 on localhost was not active within 60000ms</p>
  <p>Running <code>firebase emulators:start --only firestore</code> works fine. And if I change the command above to <code>firebase emulators:exec --only auth 'npm test'</code> the execution works as expected, but obviously the tests fail.</p>
  <p>Does anyone have an idea on how to solve this?</p>