Non-fork test ends up with unwanted scala-library even with autoScalaLibrary := false

Originally reported by @retronym 

## steps

See https://github.com/scala/scala/pull/6300#issuecomment-372495182

## problem

Classloader used by non-fork test includes scala-library in ScalaInstance.

## expectation

Construct Classloader without ScalaInstance when `autoScalaLibrary := false`