Set the default scriptedBatchExecution to true

sbt itself effectively runs its scripted test with
scriptedBatchExecution true and scriptedParallelInstances 1. The
performance is much better when this works. This can cause issues, see
https://github.com/sbt/sbt/issues/6042, but we inadvertently made this
behavior the default in 1.4.0 and it took about a month before #6042 was
reported so I think most users would benefit from this default.

main/src/main/scala/sbt/ScriptedPlugin.scala

@@ -78,7 +78,7 @@ object ScriptedPlugin extends AutoPlugin {
     scriptedClasspath := getJars(ScriptedConf).value,
     scriptedTests := scriptedTestsTask.value,
     scriptedParallelInstances := 1,
-    scriptedBatchExecution := false,
+    scriptedBatchExecution := true,
     scriptedRun := scriptedRunTask.value,
     scriptedDependencies := {
       def use[A](@deprecated("unused", "") x: A*): Unit = () // avoid unused warnings