nextcloud*: add `passthru.tests` to reference VM-tests

pkgs/servers/nextcloud/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl }:
+{ stdenv, fetchurl, nixosTests }:
 
 let
   generic = { version, sha256, insecure ? false }: stdenv.mkDerivation rec {
@@ -10,6 +10,8 @@ let
       inherit sha256;
     };
 
+    passthru.tests = nixosTests.nextcloud;
+
     installPhase = ''
       mkdir -p $out/
       cp -R . $out/