Sponsored by: The FreeBSD Foundation
The primary use case for the test is to see if WITNESS or INVARIANTS complain about something, so in that respect at least the test is useful to have.
It would be good to check that the span actually shows traffic though. I'll see if I can repurpose pft_ping.py for this. That should let us send a ping through one interface and verify that we see it on the span interface.
I think adding a test that some traffic actually passes is worthwhile, and probably a comment too about checking for WITNESS/INVARIANTS warnings (since the test does not do that programmatically).