We want to test that autodetection works with closed sockets, as we
explicitly support that scenario: chasquid might be up before dovecot
is, and we still want the detection to work.
The code is written that way, but we had no tests for it until now,
because we were blocked on the unix listeners supporting
SetUnlinkOnClose, which appeared in Go 1.8.
Now that the minimum Go version has been raised past that, we can
implement the test.