Add test for macro expansion (in Scala2)

[tanishiking]

See: https://github.com/scala/scala/tree/2.13.x/test/files/neg/warn-unused-imports

Check if there's no warn for unused import in macro expansion tree

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
 
object Macro {
  def f: Int = macro fImpl
  def fImpl(c: Context): c.Tree = {
    import c.universe._
 
    q"""
     import scala.util.Random
     42 // TODO randomize
    """
  }
}

class MacroClient {
  def x = Macro.f  // don't crash; but also don't warn on expansion, see scala/bug#10270 and [pos|neg]/t10270
}