Re-enable tests of Finalize and WeakDictionary.

Re-enable 3 tests about Finalize and WeakDictionary.

Two test cases are slightly modified to make them pass:

1. In `finalize.oz`, one action needs 2 `gcDo` instead of one to collect the new objects. I think such change is acceptable, as GC collection is not really deterministic, the only crucial test is whether the postmortem is really called.

2. Since WeakDictionary is no longer built-in, the `A.B := C` syntax used in `weakdictionaryGC.oz` is changed to `{WeakDictionary.put A B C}`.

Maybe Finalize can be modified to use WeakRef directly to revert the change in `finalize.oz`.