Replace partest neg tests with JUnit tests.

Using the ToolBox compiler. Minor cheat required:
must add header `import scala.xml.{TopScope => $scope}`,
which is normally added (synthetically) by the compiler.
Somehow, the toolbox compiler doesn't see this change to `rootContext`.
/cc @xeno-by, any idea why?

This adds a dependency on scala-compiler (for the toolbox compiler),
and on JUnit, but removes the dependency on partest.

CompilerErrors defines a class CompilerTesting that we might factor out.
It's based on https://github.com/scala/async/blob/master/src/test/scala/scala/async/package.scala#L34

review by @retronym