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/robots_txt_controller_spec.rb

@@ -49,7 +49,8 @@ RSpec.describe RobotsTxtController do
 
     context 'subfolder' do
       it 'prefixes the rules with the directory' do
-        Discourse.stubs(:base_uri).returns('/forum')
+        set_subfolder "/forum"
+
         get '/robots.txt'
         expect(response.body).to include("\nDisallow: /forum/admin")
       end