DEV: improve usability of subfolder specs

Previously people were not consistent about mocking which left internals in
a fragile state when running subfolder specs.

This introduces a simple helper `set_subfolder` which you can use to set
the subfolder for the spec. It takes care of proper configuration of subfolder
and teardown.

```
# usage
set_subfolder "/my_amazing_subfolder"
```

You should no longer stub base_uri or global_settings

spec/requests/application_controller_spec.rb

@@ -204,8 +204,7 @@ RSpec.describe ApplicationController do
       end
 
       it 'supports subfolder with permalinks' do
-        GlobalSetting.stubs(:relative_url_root).returns('/forum')
-        Discourse.stubs(:base_uri).returns("/forum")
+        set_subfolder "/forum"
 
         trashed_topic = create_post.topic
         trashed_topic.trash!