Single tenant by default & update tests

test/controllers/signups_controller_test.rb

@@ -65,17 +65,14 @@ class SignupsControllerTest < ActionDispatch::IntegrationTest
   end
 
   test "redirects to session#new when single_tenant and user exists" do
-    previous_multi_tenant = ENV["MULTI_TENANT"]
-    ENV["MULTI_TENANT"] = "false"
-
     users(:david)
 
-    untenanted do
-      get new_signup_path
+    in_single_tenant_mode do
+      untenanted do
+        get new_signup_path
 
-      assert_redirected_to new_session_url
+        assert_redirected_to new_session_url
+      end
     end
-  ensure
-    ENV["MULTI_TENANT"] = previous_multi_tenant
   end
 end