libtevent is failing tests

[frozencemetery]

Currently it's disabled in the CI, but Debian also turns off building of the tevent backend entirely, presumably for this reason.

[frozencemetery]

The issue seems to be that tevent doesn't fire the writeable callback when a file descriptor is in a bad (here: half-closed) state.

[frozencemetery]

Reported upstream to samba as #12987.

[frozencemetery]

The only way for us to work around this involves calling poll(2), which... well, we could do that, but I'd really rather not. Will wait and see what tevent does.