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/this/closure.wren

@@ -0,0 +1,7 @@
+class Foo {
+  getClosure { new Fn { toString } }
+  toString { "Foo" }
+}
+
+var closure = (new Foo).getClosure
+IO.print(closure.call()) // expect: Foo