fix: config tests without script

some tests only assert correct configuration generation, let's call
them "config tests" as opposed to "runtime tests"/"e2e tests".

Those configuration tests, don't expose a runtime script. So they
should not be taken into account for any orchestrated runtime script.

modules/testing/test-options.nix

@@ -50,6 +50,7 @@ in {
     script = mkOption {
       description = "Test script to use for e2e test";
       type = types.nullOr (types.either types.lines types.path);
+      default = null;
     };
 
   };