Add coverage that validates _async_update_matching_records

This was running in the old lots_of_names (https://github.com/jstasiak/python-zeroconf/pull/764) test but wasn't actually being
asserted.  We should have an explicit test for this path.


https://app.codecov.io/gh/jstasiak/python-zeroconf/compare/764/changes#D1L396