update test to hit default page instead of login

With Django 1.6, the test are failing to load the login page
with 'openstack_auth.user.Token object is not JSON serializable'
error.

Partially-implements blueprint django-1point6

Change-Id: I8b3d62f3076d6d2071ba77848b83e1872ba69ba6

openstack_dashboard/test/test_plugins/panel_tests.py

@@ -50,7 +50,7 @@ class PanelPluginTests(test.TestCase):
         base.Horizon._urls()
         # Trigger discovery, registration, and URLconf generation if it
         # hasn't happened yet.
-        self.client.get(settings.LOGIN_URL)
+        self.client.get("/")
 
     def tearDown(self):
         super(PanelPluginTests, self).tearDown()