Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

test/language/closure/reuse_closure_slot.wren

@@ -0,0 +1,17 @@
+{
+  var f = null
+
+  {
+    var a = "a"
+    f = new Fn { IO.print(a) }
+  }
+
+  {
+    // Since a is out of scope, the local slot will be reused by b. Make sure
+    // that f still closes over a.
+    var b = "b"
+    f.call() // expect: a
+  }
+}
+
+// TODO: Maximum number of closed-over variables (directly and/or indirect).