Test the callbacks in rdf.mapping.node.forum.yml

Problem/Motivation

This is a followup from #2090139: Warning: Illegal string offset 'callable' in rdf_rdfa_attributes() (line 181 of core/modules/rdf/rdf.module). → . That issue was committed to 8.x but a test was not added at the time.

Steps to reproduce

See #2090139: Warning: Illegal string offset 'callable' in rdf_rdfa_attributes() (line 181 of core/modules/rdf/rdf.module). → .

Proposed resolution

Write a test

Remaining tasks

Write a test
Review

User interface changes

API changes

Data model changes

Release notes snippet