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/field/closure.wren

@@ -0,0 +1,16 @@
+class Foo {
+  new { _field = "Foo field" }
+
+  closeOverGet {
+    return new Fn { _field }
+  }
+
+  closeOverSet {
+    return new Fn { _field = "new value" }
+  }
+}
+
+var foo = new Foo
+IO.print(foo.closeOverGet.call()) // expect: Foo field
+foo.closeOverSet.call()
+IO.print(foo.closeOverGet.call()) // expect: new value