Revived tests that once depended on xml

I was a bit overzealous in moving stuff over to scala-xml in 9c50dd5274
These were all compiler tests that accidentally touched on xml.
I've tried to delicately decouple them so they can roam the
scalac pastures as intended.
Merge pull request #3135 from adriaanm/revive-xml-tests

Revived tests that once depended on xml