Undo JVM forking in test, reverting scala#112

* build.sbt (fork): SBT was leaking scala-xml from scala-compiler
because of an incorrect library dependency exclude rule.

Author: ashawley

build.sbt

@@ -39,8 +39,3 @@ libraryDependencies += "org.scalacheck" %% "scalacheck" % "1.13.2" % "test"
 libraryDependencies += ("org.scala-lang" % "scala-compiler" % scalaVersion.value % "test").exclude("org.scala-lang.modules", s"scala-xml_${scalaVersion.value}")
 
 mimaPreviousVersion := Some("1.0.5")
-
-// You cannot disable JVM test forking when working on scala modules
-// that are distributed with the compiler because of an SBT
-// classloader leaking issue (scala/scala-xml#20 and #112).
-fork in Test := true