Render custom tag briefs for enum entries and constructor

[strangesource]

Custom tags brief not rendered for enum entries and constructors

Contrary to what the documentation on the CustomTagContentProvider suggests, the custom tag brief is not rendered for constructors and enum entries. Reported in #4255.

Changes

• Render content for custom tag briefs for enum entries and constructors
• Minimal unit test

Issues encountered

While setting up the unit tests i struggled with the assertion DSL. It doesn't seem fight to assert the whole page tree until the node I actually want to ensure is present, maybe you have some suggestion on how this could be improved.