Do not load openstack_dashboard.settings in unit tests

The previous behavior depends on different values for
SELECTABLE_THEMES and it seems this is the reason that test_themes
imported openstack_dashboard.settings.
If we override a value in django.conf.settings, we can use
override_settings decorator. This commit uses it and
we can now avoid importing openstack_dashboard.settings in UT.

Change-Id: Id1b7b7e431b86e725ad897613ca38f423a67e52d
Closes-Bug: #1809842

openstack_dashboard/test/settings.py

@@ -21,6 +21,7 @@ from horizon.utils import secret_key
 
 from openstack_dashboard import enabled
 from openstack_dashboard import exceptions
+from openstack_dashboard import theme_settings
 from openstack_dashboard.utils import settings as settings_utils
 
 # this is used to protect from client XSS attacks, but it's worth
@@ -60,14 +61,8 @@ AVAILABLE_THEMES = [
         'themes/material'
     ),
 ]
-
-SELECTABLE_THEMES = [
-    (
-        'default',
-        pgettext_lazy('Default style theme', 'Default'),
-        'themes/default'
-    ),
-]
+AVAILABLE_THEMES, SELECTABLE_THEMES, DEFAULT_THEME = \
+    theme_settings.get_available_themes(AVAILABLE_THEMES, 'default', None)
 
 # Theme Static Directory
 THEME_COLLECTION_DIR = 'themes'

openstack_dashboard/test/unit/test_themes.py

@@ -10,7 +10,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from openstack_dashboard import settings
+from django.conf import settings
+from django.test.utils import override_settings
+
 from openstack_dashboard.templatetags import themes
 from openstack_dashboard.test import helpers as test
 
@@ -23,6 +25,9 @@ class SelectableThemeTest(test.TestCase):
         # 'available' list
         self.assertEqual(selectable, available)
 
+    @override_settings(SELECTABLE_THEMES=[
+        ('default', 'Default', 'themes/default'),
+    ])
     def test_selectable_override(self):
         selectable = themes.themes()
         available = themes.settings.AVAILABLE_THEMES