Encoding an empty array or object worked, but encoding it again
(possibly after adding some items) failed, because the visited flag
(used for detecting circular references) wasn't zeroed.
* Now that JSON_SORT_KEYS is implemented, take it into use with the
valid and valid-strip suites. This is to ensure that the tests
remain valid even if the string hash function is changed in the
future.
* Remove test_dump API test. Instead, implement the same tests more
elegantly in the encoding-flags suite.
