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

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
How about doing the below things?
```
fakeTextSelectionApp.copy('centerInput');
fakeTextSelectionApp.paste('rightInput');
assert.equal(
  fakeTextSelectionApp.centerInput,
  fakeTextSelectionApp.rightInput
);
```