Test locally built scala parser-combinators classes.

Somehow, the scala-parser-combinators classes used to run tests were
not the local ones, even though there is no dependency on scala-compiler
nor on anything that depends on scala-parser-combinators.

After some head-scratching, I discovered scala/scala-xml#20. Now I am even
more confused, but at least `fork in Test := true` does prevent this
evil classloader magic.