Restore old scripted behavior with batch tests disabled

When 1 is passed in as the sbtInstances argument to runInParallel, all
of the tests get batched together in a single sbt session. The run
methods in ScriptedRunner were delegating to runInParallel and as a
result were causing all of the tests to be batched, which was not how it
used to work in sbt 1.3.x. To fix this we can instead pass in
Int.MaxValue for the number of sbt instances. The Int.MaxValue parameter
causes the batch size to be set to 1 in the batchScriptedRunner method
which causes the scriptedRunners variable to have the same size as the
number of tests. We then can prevent a parallel array from being used if
the sbtInstances is deteced to be the Int.MaxValue sentinel.

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

@@ -376,7 +376,7 @@ class ScriptedRunner {
       launchOpts: Array[String],
   ): Unit = {
     val logger = TestConsoleLogger()
-    runInParallel(
+    run(
       resourceBaseDirectory,
       bufferLog,
       tests,
@@ -384,7 +384,8 @@ class ScriptedRunner {
       launchOpts,
       prescripted = new java.util.ArrayList[File],
       LauncherBased(launcherJar),
-      1
+      1,
+      parallelExecution = false,
     )
   }
 
@@ -403,7 +404,7 @@ class ScriptedRunner {
       prescripted: java.util.List[File],
   ): Unit = {
     val logger = TestConsoleLogger()
-    runInParallel(
+    run(
       resourceBaseDirectory,
       bufferLog,
       tests,
@@ -411,7 +412,8 @@ class ScriptedRunner {
       launchOpts,
       prescripted,
       LauncherBased(launcherJar),
-      1
+      Int.MaxValue,
+      parallelExecution = false,
     )
   }
 
@@ -477,6 +479,18 @@ class ScriptedRunner {
       prescripted: java.util.List[File],
       prop: RemoteSbtCreatorProp,
       instances: Int
+  ) = run(baseDir, bufferLog, tests, logger, launchOpts, prescripted, prop, instances, true)
+
+  private[this] def run(
+      baseDir: File,
+      bufferLog: Boolean,
+      tests: Array[String],
+      logger: Logger,
+      launchOpts: Array[String],
+      prescripted: java.util.List[File],
+      prop: RemoteSbtCreatorProp,
+      instances: Int,
+      parallelExecution: Boolean,
   ): Unit = {
     val addTestFile = (f: File) => { prescripted.add(f); () }
     val runner = new ScriptedTests(baseDir, bufferLog, launchOpts)
@@ -490,11 +504,17 @@ class ScriptedRunner {
     // The scripted tests mapped to the inputs that the user wrote after `scripted`.
     val scriptedTests =
       get(tests, baseDir, accept, logger).map(st => (st.group, st.name))
+    // Choosing Int.MaxValue will make the groupSize 1 in batchScriptedRunner
+    val groupCount = if (parallelExecution) instances else Int.MaxValue
     val scriptedRunners =
-      runner.batchScriptedRunner(scriptedTests, addTestFile, instances, prop, logger)
-    val parallelRunners = scriptedRunners.toParArray
-    parallelRunners.tasksupport = new ForkJoinTaskSupport(new ForkJoinPool(instances))
-    runAll(parallelRunners)
+      runner.batchScriptedRunner(scriptedTests, addTestFile, groupCount, prop, logger)
+    if (parallelExecution && instances > 1) {
+      val parallelRunners = scriptedRunners.toParArray
+      parallelRunners.tasksupport = new ForkJoinTaskSupport(new ForkJoinPool(instances))
+      runAll(parallelRunners)
+    } else {
+      runAll(scriptedRunners)
+    }
   }
   def runInParallel(
       baseDir: File,