Bug 1559992 - [marionette] Fix test_modal_dialogs.py TestTabModalAlerts.test_handle_two_dialogs. r=marionette-reviewers,jgraham

The second alert is not expected to immediately be created by the
browser. As such the test has to wait for it.

Differential Revision: https://phabricator.services.mozilla.com/D105317

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

@@ -148,6 +148,8 @@ class TestTabModalAlerts(BaseAlertTestCase):
         alert1.send_keys("foo")
         alert1.accept()
 
+        self.wait_for_alert()
+
         alert2 = self.marionette.switch_to_alert()
         alert2.send_keys("bar")
         alert2.accept()