> I tried to find a way of avoiding changing global state in my test -
> but because of the problem I mention, I couldn't see a way of using
> smtpd without affecting global state.

Well, other tests manage it even without using a private socket map.
Leaving dangling sockets in the socket map could mean your code forgets
to close them, for example. So it's better to fix the code or the tests,
rather than try to defeat the detection mechanism.