Tests: Modify viewport size for parity between dev and CI env

In CI we are getting MoveTargetOutOfBoundsException, but not locally.
We have had errors in the past due to different viewport sizes in CI
and locally, so setting this to a standard size for the pages layout
tests (where the exception is occurring).

(cherry picked from commit ed3bb91)

securedrop/tests/functional/functional_test.py

@@ -319,6 +319,12 @@ def start_journalist_server(app):
         else:
             # We will use a normal firefox esr for the pages-layout tests
             self.driver = self._create_webdriver2(self.new_profile)  # noqa # pylint: disable=no-member
+
+            # Set window size and position explicitly to avoid potential bugs due
+            # to discrepancies between environments.
+            self.driver.set_window_position(0, 0)
+            self.driver.set_window_size(1024, 768)
+
             self._javascript_toggle()
 
         # Polls the DOM to wait for elements. To read more about why