Turns out that we *did* have a test that would have shown up the
DsForeign bug (rev. 1.70 of ghc/compiler/deSugar/DsForeign.lhs) but it
was disabled because it didn't quite work with the current test
framework.

I've hacked the framework a bit and hopefully knocked the test into
shape so we shouldn't get the same problem again.

MERGE TO STABLE after testing in HEAD