Turn on forking in test JVMs to reduce the pressure on perm gen and code

cache sizes due to having 2 instances of the Scala compiler and a bunch
of classloaders.

project/SparkBuild.scala

@@ -44,6 +44,10 @@ object SparkBuild extends Build {
     transitiveClassifiers in Scope.GlobalScope := Seq("sources"),
     testListeners <<= target.map(t => Seq(new eu.henkelmann.sbt.JUnitXmlTestsListener(t.getAbsolutePath))),
 
+    // Fork new JVMs for tests and set Java options for those
+    fork := true,
+    javaOptions += "-Xmx1g",
+
     // Only allow one test at a time, even across projects, since they run in the same JVM
     concurrentRestrictions in Global += Tags.limit(Tags.Test, 1),