Add scripted test for closing classloaders

https://github.com/sbt/sbt/issues/5075 reports that sbt 1.3.0 has a
regression for in process run. In sbt 1.3.0, we close the classloader
that invokes the main method before all of the daemon threads complete.
This adds a scripted test that fails due to this regression.

sbt/src/sbt-test/classloader-cache/close-run/src/main/scala/Main.scala

@@ -0,0 +1,10 @@
+object Main extends App {
+  class Foo
+  new Thread {
+    override def run(): Unit = {
+      Thread.sleep(500)
+      try new Foo
+      catch { case t: Throwable => sys.exit(1) }
+    }
+  }.start()
+}

sbt/src/sbt-test/classloader-cache/close-run/test

@@ -0,0 +1 @@
+> run