fix: Java-only tests

**Problem**
managedScalaInstance := false fails Java-only tests because
the test runner is based on a Scala Runner and sbt 2.x
currently derives the ScalaInstance only from update.

**Solution**
For the purpose of Java-only testing, create a dummy Scala Instance.

main/src/main/scala/sbt/internal/Compiler.scala

@@ -19,14 +19,32 @@ object Compiler:
       val sh = Keys.scalaHome.value
       val app = Keys.appConfiguration.value
       val sv = Keys.scalaVersion.value
+      val managed = Keys.managedScalaInstance.value
       sh match
         case Some(h) => scalaInstanceFromHome(h)
         case _ =>
           val scalaProvider = app.provider.scalaProvider
-          scalaInstanceFromUpdate
+          if !managed then emptyScalaInstance
+          else scalaInstanceFromUpdate
     }
 
-  // use the same class loader as the Scala classes used by sbt
+  /**
+   * A dummy ScalaInstance for Java-only projects.
+   */
+  def emptyScalaInstance: Def.Initialize[Task[ScalaInstance]] = Def.task {
+    makeScalaInstance(
+      "0.0.0",
+      Array.empty,
+      Seq.empty,
+      Seq.empty,
+      Keys.state.value,
+      Keys.scalaInstanceTopLoader.value,
+    )
+  }
+
+  // Use the same class loader as the Scala classes used by sbt
+  // This will fail for "doc" task https://github.com/sbt/sbt/issues/7725
+  // because Scala 3 uses ScalaDocTool configuration to manage doc tools.
   def optimizedScalaInstance(
       sv: String,
       scalaProvider: ScalaProvider

sbt-app/src/sbt-test/tests/junit-java-only/build.sbt

@@ -0,0 +1,2 @@
+libraryDependencies += "com.github.sbt" % "junit-interface" % "0.13.2" % Test
+managedScalaInstance := false

sbt-app/src/sbt-test/tests/junit-java-only/src/test/java/com/example/ATest.java

@@ -0,0 +1,9 @@
+package com.example;
+
+import org.junit.Test;
+
+public final class ATest {
+  @Test
+  public void testSomething() {
+  }
+}

sbt-app/src/sbt-test/tests/junit-java-only/test

@@ -0,0 +1,2 @@
+> test
+> doc

sbt-app/src/sbt-test/tests/junit/build.sbt

@@ -1 +1 @@
-libraryDependencies += "com.novocode" % "junit-interface" % "0.8" % "test"
+libraryDependencies += "com.github.sbt" % "junit-interface" % "0.13.2" % Test