Fixed a couple filter tests. - a length check should come before the access, so you can verify whether the access will succeed.
Fixed a couple filter tests.
- a length check should come before the access, so you can verify
whether the access will succeed.

- Also removed some tests which can't work, due to the filter
changes.  In fact, a lot of these tests should probably be
removed or changed if we want to disallow running
apply_common_filters() on plain dicts.  They will often
coincidentally still succeed though, so I left them in.

oasis-open/cti-python-stix2