Improve test failure message for #1390.

util/classpath/src/test/scala/sbt/classpath/ConcurrentCache.scala

@@ -9,11 +9,20 @@ object ConcurrentCache extends Properties("ClassLoaderCache concurrent access")
   implicit lazy val concurrentArb: Arbitrary[Int] = Arbitrary(Gen.choose(1, 1000))
   implicit lazy val filenameArb: Arbitrary[String] = Arbitrary(Gen.alphaStr)
 
+  private[this] object DifferentClassloader {
+    def unapply(loaders: Seq[ClassLoader]): Option[(ClassLoader, ClassLoader)] =
+      if (loaders.size > 1) loaders.sliding(2).find { case Seq(x, y) => x != y } map { case Seq(x, y) => (x, y) }
+      else None
+  }
+
   property("Same class loader for same classpaths concurrently processed") = forAll { (names: List[String], concurrent: Int) =>
     withcp(names.distinct) { files =>
       val cache = new ClassLoaderCache(null)
       val loaders = (1 to concurrent).par.map(_ => cache(files)).toList
-      sameClassLoader(loaders)
+      loaders match {
+        case DifferentClassloader(left, right) => false :| s"$left != $right"
+        case _                                 => true :| ""
+      }
     }
   }
 
@@ -25,6 +34,4 @@ object ConcurrentCache extends Properties("ClassLoaderCache concurrent access")
     }
     f(files)
   }
-  private[this] def sameClassLoader(loaders: Seq[ClassLoader]): Boolean = loaders.size < 2 ||
-    loaders.sliding(2).forall { case Seq(x, y) => x == y }
 }