Allow sbt scripted tests to run in parallel

I'm not sure if this is a huge benefit or not, but it's nice to have the
option to run the scripted tests in parallel. The default behavior
should be the same as before.

project/Scripted.scala

@@ -115,6 +115,7 @@ object Scripted {
           bootProperties: File,
           launchOpts: Array[String],
           prescripted: java.util.List[File],
+          instances: Int
       ): Unit
     }
 
@@ -123,7 +124,6 @@ object Scripted {
       Thread.currentThread.setContextClassLoader(loader)
       val bridge =
         bridgeClass.getDeclaredConstructor().newInstance().asInstanceOf[SbtScriptedRunner]
-
       try {
         // Using java.util.List to encode File => Unit.
         val callback = new java.util.AbstractList[File] {
@@ -131,6 +131,13 @@ object Scripted {
           def get(x: Int): sbt.File = ???
           def size(): Int = 0
         }
+        val instances: Int = (System.getProperty("sbt.scripted.parallel.instances") match {
+          case null => 1
+          case i    => scala.util.Try(i.toInt).getOrElse(1)
+        }) match {
+          case i if i > 0 => i
+          case _          => 1
+        }
         import scala.language.reflectiveCalls
         bridge.runInParallel(
           sourcePath,
@@ -139,6 +146,7 @@ object Scripted {
           launcher,
           launchOpts.toArray,
           callback,
+          instances
         )
       } catch { case ite: InvocationTargetException => throw ite.getCause }
     } finally {

scripted-sbt-redux/src/main/scala/sbt/scriptedtest/ScriptedTests.scala

@@ -125,7 +125,10 @@ final class ScriptedTests(
     if (labelsAndDirs.isEmpty) List()
     else {
       val totalSize = labelsAndDirs.size
-      val batchSize = totalSize / sbtInstances
+      val batchSize = totalSize / sbtInstances match {
+        case 0 => 1
+        case s => s
+      }
 
       val (launcherBasedTests, runFromSourceBasedTests) = labelsAndDirs.partition {
         case (testName, _) =>