Don't leak the sbt boot scala library into tests

It was reported in https://github.com/sbt/sbt/issues/4608 that there was
a regression that tests run against scala 2.11 would fail. This was
because the interface loader incorrectly contained the scala library. To
fix this, I needed to find the xsbt.boot.BootFilteredLoader in the
classloading hierarchy and put the sbt testing interface library in
between that loader and the scala library loader.

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

@@ -21,6 +21,8 @@ import sbt.internal.util.Attributed.data
 import sbt.io.IO
 import sbt.librarymanagement.Configurations.{ Runtime, Test }
 
+import scala.annotation.tailrec
+
 private[sbt] object ClassLoaders {
   private[this] val interfaceLoader = classOf[sbt.testing.Framework].getClassLoader
   /*
@@ -214,9 +216,15 @@ private[sbt] object SbtMetaBuildClassLoader {
     }
   }
   def apply(libraryLoader: ClassLoader, fullLoader: ClassLoader): ClassLoader = {
+    @tailrec
+    def bootLoader(classLoader: ClassLoader): ClassLoader = classLoader.getParent match {
+      case null                                                               => classLoader
+      case c if c.getClass.getCanonicalName == "xsbt.boot.BootFilteredLoader" => c
+      case c                                                                  => bootLoader(c)
+    }
     val interfaceFilter: URL => Boolean = _.getFile.endsWith("test-interface-1.0.jar")
     val (interfaceURL, rest) = fullLoader.urls.partition(interfaceFilter)
-    val interfaceLoader = new URLClassLoader(interfaceURL, libraryLoader.getParent) {
+    val interfaceLoader = new URLClassLoader(interfaceURL, bootLoader(libraryLoader)) {
       override def toString: String = s"SbtTestInterfaceClassLoader(${getURLs.head})"
     }
     val updatedLibraryLoader = new URLClassLoader(libraryLoader.urls, interfaceLoader) {

sbt/src/sbt-test/tests/test-cross/build.sbt

@@ -0,0 +1,8 @@
+val scalatest = "org.scalatest" %% "scalatest" % "3.0.5"
+
+lazy val root = (project in file("."))
+  .settings(
+    // Verifies that a different scala library version still works in test
+    scalaVersion := "2.11.12",
+    libraryDependencies += scalatest % Test
+  )

sbt/src/sbt-test/tests/test-cross/src/test/scala/Test.scala

@@ -0,0 +1,8 @@
+import java.io.File
+import org.scalatest.FlatSpec
+
+class Test extends FlatSpec {
+  "a test" should "pass" in {
+    assert(true)
+  }
+}

sbt/src/sbt-test/tests/test-cross/test

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

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

@@ -255,7 +255,9 @@ final class ScriptedTests(
       case "source-dependencies/linearization"                      => LauncherBased // sbt/Package$
       case "source-dependencies/named"                              => LauncherBased // sbt/Package$
       case "source-dependencies/specialized"                        => LauncherBased // sbt/Package$
-      case _                                                        => RunFromSourceBased
+      case "tests/test-cross" =>
+        LauncherBased // the sbt metabuild classpath leaks into the test interface classloader in older versions of sbt
+      case _ => RunFromSourceBased
     }
     // sbt/Package$ means:
     //   java.lang.NoClassDefFoundError: sbt/Package$ (wrong name: sbt/package$)