Bug 1381876 - Fix window maximised assertions. r=automatedtester

The "delta" in the second assertion was not deducted from the height.
The patch also improves the error message.

MozReview-Commit-ID: 8u8UhKg3Q62

--HG--
extra : rebase_source : a3771620fc15225ac6932c718515b8dfcf2fdab0

testing/marionette/harness/marionette_harness/tests/unit/test_window_maximize.py

@@ -39,13 +39,13 @@ class TestWindowMaximize(MarionetteTestCase):
         self.assertGreaterEqual(
             actual["width"], self.max["width"] - delta,
             msg="Window width is not within {delta} px of availWidth: "
-                "current width {expected} should be greater than max width {max}"
-                .format(delta=delta, expected=actual["width"], max=self.max["width"] - delta))
+                "current width {current} should be greater than or equal to max width {max}"
+                .format(delta=delta, current=actual["width"], max=self.max["width"] - delta))
         self.assertGreaterEqual(
-            actual["height"], self.max["height"],
+            actual["height"], self.max["height"] - delta,
             msg="Window height is not within {delta} px of availHeight: "
-                "current height {expected} should be greater than max width {max}"
-                .format(delta=delta, expected=actual["height"], max=self.max["height"] - delta))
+                "current height {current} should be greater than or equal to max height {max}"
+                .format(delta=delta, current=actual["height"], max=self.max["height"] - delta))
 
     def assert_window_restored(self, actual):
         self.assertEqual(self.original_size["width"], actual["width"])