When trying to debug why the tests broke for
https://github.com/MichMich/MagicMirror/pull/2946 I found that the tests
does not wait for the app to start and close. So if the startup isn't
blocking that would fail.
So I added a callback for `close()` too and converted them to promises
for the `startApplication()` and `stopApplication()` and updated all the
e2e tests to await both. Will try to refactor all these callbacks to
promises in a later PR.
