Complete delete backend regtests by checking deletion of a proxy with a
reference on an unnamed defaults instance. This operation is sensible as
the defaults refcount is decremented, and when the last backend is
removed, the defaults is also freed.
Add a reg-tests to test "del backend" CLI command. First, checks are
performed to ensure a backend cannot be deleted if not in the expected
state.
Then, a "del backend" success is tested. Stats are dumped to ensure the
backend instance is indeed removed.
