Merge pull request #291916 from K900/more-version-pins

nixos/lib/testing: remove yet another source of unnecessary test rebuilds

nixos/lib/testing/nixos-test-base.nix

···
       # version number.
       config.system.nixos = {
         revision = mkForce "constant-nixos-revision";
+        versionSuffix = mkForce "test";
         label = mkForce "test";
       };
     }