Fix dynamic controller argument deprecation warning.

Be gone!

```
DEPRECATION WARNING: Using a dynamic :controller segment in a route is deprecated and will be removed in Rails 5.2. (called from block in setup at /Users/kasperhansen/Documents/code/rails-observers/test/sweeper_test.rb:33)
```

Author: kaspth

test/sweeper_test.rb

@@ -30,7 +30,7 @@ def setup
     @routes = SharedTestRoutes
 
     @routes.draw do
-      get ':controller(/:action)'
+      get 'sweeper_test/show'
     end
 
     super