The tests TLS setup was bogus: the xmpp-send-message script was trying to connect to the server through a bogus domain name. Injecting the right one. I'm a bit confused about that one. I know for sure this NixOS test succeeded last time I checked it, but the TLS conf is bogus for sure. I assume the slixmpp SNI validation was a bit too loose and was tightened at some point.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at https://nixos.org/nixos and in the manual in doc/manual.