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/categories_controller_spec.rb

@@ -21,8 +21,7 @@ describe CategoriesController do
     end
 
     it 'web crawler view has correct urls for subfolder install' do
-      GlobalSetting.stubs(:relative_url_root).returns('/forum')
-      Discourse.stubs(:base_uri).returns("/forum")
+      set_subfolder "/forum"
       get '/categories', headers: { 'HTTP_USER_AGENT' => 'Googlebot' }
       html = Nokogiri::HTML(response.body)
       expect(html.css('body.crawler')).to be_present