Problem

With Test / autoScalaLibrary := false, the non-fork test classloader still included the ScalaInstance-based scala-library layer. Tests could load the wrong scala-library (e.g. sbt's) and hit NoSuchMethodError or version mismatch (e.g. scala/scala had to use fork in Test := true as a workaround).

Solution

In ClassLoaders.testTask:

• When autoScalaLibrary.value is false, use ClassLoaderLayeringStrategy.Flat so the test classloader is built without the ScalaInstance layer.
• Use project classpath only (rawCP) for fullCP in that case (do not prepend si.libraryJars).
• Do not add si.libraryJars to the exclude set when autoScalaLibrary is false.

No change to forked tests or to the run task.

Verification

1. Reproduce (without fix): In a project with Test / autoScalaLibrary := false and a test framework that pulls in Scala (e.g. ScalaCheck), run test without forking; previously the framework could load scala-library from the ScalaInstance layer and fail (e.g. NoSuchMethodError: scala.collection.immutable.Set$.apply).
2. Verify: Same project runs test successfully with this fix (test classloader uses only project classpath).
3. Scripted: ./sbt "scripted tests/autoScalaLibrary-test-loader" — runs tests with Test / autoScalaLibrary := false.
4. Regression: ./sbt "scripted classloader-cache/scalatest" — passes.

Tests / lint

• ./sbt compile — success
• ./sbt "scripted tests/autoScalaLibrary-test-loader" — success
• ./sbt "scripted classloader-cache/scalatest" — success

Closes #4009