Enable all disabled tests, add a few more including some with null documents.

Essentially, this change adds the test cases from this repo:
   https://github.com/json-patch/json-patch-tests

Specifically:
   https://github.com/json-patch/json-patch-tests/blob/master/spec_tests.json
   https://github.com/json-patch/json-patch-tests/blob/master/tests.json

The files were taken at the date of this commit, at git hash
  ea3af85790

Some tests may not have an 'expected' or 'error' field. Those are ignored.
One test was disabled manually via "disabled_in_json_c", because it tries
an impossible test, i.e. to add 2 "op" fields in the same patch entry,
something which is impossible in a JSON object.

For the 'error' cases, right now we only test that an error happens.
Later, we can extend this to check the error codes.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>