Set the browser size on startup of acceptance tests.

This resolves an issue on Mac OS X in which the login dropdown
arrow couldn't be clicked because it was off-screen.
It should make the test suite more stable.

common/djangoapps/terrain/browser.py

@@ -74,6 +74,9 @@ def initial_setup(server):
     if not success:
         raise IOError("Could not acquire valid ChromeDriver browser session.")
 
+    # Set the browser size to 1280x1024
+    world.browser.driver.set_window_size(1280, 1024)
+
 
 @before.each_scenario
 def reset_data(scenario):