They consistently fail since openjdk bump with some out-of-space errors. That's not a problem by itself, but each test instance ties a build slot for many hours and consequently they also delay channels as those wait for all builds to finish. Feel free to re-enable when fixed, of course.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.