move nixos-test to integration-test.nix

(cherry picked from commit fa813f6216)

flake.nix

@@ -220,7 +220,7 @@
               overlays = [ self.overlays.default ];
             };
             certs = import "${nixpkgs}/nixos/tests/common/acme/server/snakeoil-certs.nix";
-            test = import ./nixos-test.nix { inherit certs; };
+            test = import ./integration-test.nix { inherit certs; };
           in
           pkgsMobilizon.nixosTest test;
       });