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

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
I think we should use same logic[1][2] `<` to compare the locations.
[1] https://github.com/mozilla-b2g/gaia/pull/20910/files#diff-1b27bbfeebcd899b3619e0bbf1767af3R33
[2] https://github.com/mozilla-b2g/gaia/pull/20910/files#diff-1b27bbfeebcd899b3619e0bbf1767af3R44