fix for the test_add_delete_category_for_package

Found that selenium kept focus on modal dialog window when action
go_to_submenu() was called. So the menu element wasn't clickable.
This is a timing issue. Just added wait.

Change-Id: I3756089e1e08b2a29a42336cb781becd01cf6be1
Closes-Bug: #1545540

muranodashboard/tests/functional/sanity_check.py

@@ -1246,6 +1246,10 @@ class TestSuitePackageCategory(base.PackageTestCase):
 
         self.wait_for_alert_message()
 
+        # To wait till the focus is swithced
+        # from modal dialog back to the window.
+        self.wait_for_sidebar_is_loaded()
+
     def setUp(self):
         super(TestSuitePackageCategory, self).setUp()