Clarify test for no _project_name

We now provide other data, and still want to be sure that the
test passes.
Insist that all bugs coming in must have last_polled

This is a timestamp that lets us diagnose if a Bug object is stale.
Therefore, it is essential that we get this data from what we import.

This commit adds the validation code to core_importers.py and adds
a test.
Merge remote-tracking branch 'origin/require_last_polled'