[2.x] fix: Fixes autoScalaLibrary in test classloader (#8776)

When Test / autoScalaLibrary := false, build the non-fork test classloader
without the ScalaInstance layer: use Flat strategy and project classpath
only (rawCP), so tests do not see sbt's scala-library and avoid
NoSuchMethodError / version mismatch (e.g. scala/scala build).

- ClassLoaders.testTask: read autoScalaLibrary; when false use Flat and
  rawCP-only fullCP, and do not add si.libraryJars to exclude.
- Add scripted test tests/autoScalaLibrary-test-loader that runs test
  with Test / autoScalaLibrary := false.

main/src/main/scala/sbt/internal/ClassLoaders.scala

@@ -38,6 +38,7 @@ private[sbt] object ClassLoaders {
    */
   private[sbt] def testTask: Def.Initialize[Task[ClassLoader]] = Def.task {
     val si = scalaInstance.value
+    val useScalaLibraryLayer = autoScalaLibrary.value
     val converter = fileConverter.value
     val cp = fullClasspath.value
       .map(_.data)
@@ -47,17 +48,21 @@ private[sbt] object ClassLoaders {
     def getLm(f: File): Long = dependencyStamps.getOrElse(f, IO.getModifiedTimeOrZero(f))
     val rawCP = cp.map(f => f -> getLm(f))
     val fullCP =
-      if si.isManagedVersion then rawCP
+      if !useScalaLibraryLayer then rawCP
+      else if si.isManagedVersion then rawCP
       else si.libraryJars.map(j => j -> IO.getModifiedTimeOrZero(j)).toSeq ++ rawCP
     val exclude: Set[File] = dependencyJars(exportedProducts).value
       .map(converter.toPath)
       .map(_.toFile)
-      .toSet ++ si.libraryJars
+      .toSet ++ (if useScalaLibraryLayer then si.libraryJars.toSeq else Seq.empty)
+    val strategy =
+      if useScalaLibraryLayer then classLoaderLayeringStrategy.value
+      else ClassLoaderLayeringStrategy.Flat
     val logger = state.value.globalLogging.full
     val close = closeClassLoaders.value
     val allowZombies = allowZombieClassLoaders.value
     buildLayers(
-      strategy = classLoaderLayeringStrategy.value,
+      strategy = strategy,
       si = si,
       fullCP = fullCP,
       allDependenciesSet = dependencyJars(dependencyClasspath).value

sbt-app/src/sbt-test/tests/autoScalaLibrary-test-loader/build.sbt

@@ -0,0 +1,5 @@
+ThisBuild / scalaVersion := "2.12.21"
+
+Test / autoScalaLibrary := false
+libraryDependencies += "org.scala-lang" % "scala-library" % scalaVersion.value % Test
+libraryDependencies += "org.scalatest" %% "scalatest" % "3.2.19" % Test

sbt-app/src/sbt-test/tests/autoScalaLibrary-test-loader/src/test/scala/ExampleSpec.scala

@@ -0,0 +1,8 @@
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class ExampleSpec extends AnyFlatSpec with Matchers {
+  "test with autoScalaLibrary false" should "use project classpath only" in {
+    1 shouldBe 1
+  }
+}

sbt-app/src/sbt-test/tests/autoScalaLibrary-test-loader/test

@@ -0,0 +1 @@
+> test