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

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
For the `end` param, we should follow the design of `substring` method[1].
```
end: Optional. The position (up to, but not including) where to end the extraction. If omitted, it extracts the rest of the string
```
[1] http://www.w3schools.com/jsref/jsref_substring.asp