Bug 1023087 - [Window Management] Marionette test for copy-paste

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
```
assert.ok(
  fakeTextselectionApp.centerInput.location.y >
  fakeTextselectionApp.textSelection.position.y,
  'dialog should be placed higher than the input field'
);
```