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/nonlocal/mutual_recursion.wren

@@ -0,0 +1,10 @@
+class Foo {
+  static bar { new Bar }
+}
+
+class Bar {
+  static foo { new Foo }
+}
+
+IO.print(Foo.bar) // expect: instance of Bar
+IO.print(Bar.foo) // expect: instance of Foo