SPARK-3151: increase max heap size for forked tests.

Author: Eyal Farago

project/SparkBuild.scala

@@ -790,7 +790,7 @@ object TestSettings {
     javaOptions in Test ++= System.getProperties.asScala.filter(_._1.startsWith("spark"))
       .map { case (k,v) => s"-D$k=$v" }.toSeq,
     javaOptions in Test += "-ea",
-    javaOptions in Test ++= "-Xmx3g -Xss4096k"
+    javaOptions in Test ++= "-Xmx6g -Xss4096k"
       .split(" ").toSeq,
     javaOptions += "-Xmx3g",
     // Exclude tags defined in a system property