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/static_field/use_before_set.wren

@@ -0,0 +1,8 @@
+class Foo {
+  static write { IO.print(__field) } // Compile a use of the field...
+  static init { __field = "value" }  // ...before an assignment to it.
+}
+
+// But invoke them in the right order.
+Foo.init
+Foo.write // expect: value