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/use_before_set.wren

@@ -0,0 +1,9 @@
+class Foo {
+  write { IO.print(_field) } // Compile a use of the field...
+  init { _field = "value" }  // ...before an assignment to it.
+}
+
+var foo = new Foo
+// But invoke them in the right order.
+foo.init
+foo.write // expect: value