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

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
->
```
test('click top-left input', function() {
  fakeTextselectionApp.press('topLeftInput');
  var textSelectionLocation = fakeTextselectionApp.textSelection.location

  assert.ok(
    fakeTextselectionApp.topLeftInput.location <
    textSelectionLocation.y,
    'dialog should be placed lower than the input field'
  );
  assert.equal(
    textSelectionLocation.x,
    0,
    'dialog should be placed near left boundary'
  );
});
```