Fix issues with specifying scalaHome/scalaInstance and running tests

Fixes an issue where the classpath filter was blocking loading Scala classes inside test-runners, thanks to an invalid classpath list.

cc> @paulp Any chance you've seen this issue?  I just discovered it the other day, but given your usage of sbt, I think you may be the only one I know who would have hit this, may be able to validate it outside the Akka build/test case.

Review by @cunei cc> @eed3si9n @rkuhn