tests: discard const in test_confdb_get_enabled_domain_list

```
../../src/tests/cmocka/confdb/test_confdb.c:247:51: error: passing argument 2 of ‘string_in_list’ from incompatible pointer type [-Werror=incompatible-pointer-types]
  247 |         assert_true(string_in_list(result[index], expected_enabled_domain_list, false));
      |                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                   |
      |                                                   const char **
```

src/tests/cmocka/confdb/test_confdb.c

@@ -244,7 +244,7 @@ static void test_confdb_get_enabled_domain_list(void **state)
         assert_true(string_in_list(expected_enabled_domain_list[index], result, false));
     }
     for (int index = 0; result[index]; index++) {
-        assert_true(string_in_list(result[index], expected_enabled_domain_list, false));
+        assert_true(string_in_list(result[index], discard_const(expected_enabled_domain_list), false));
     }
 
     TALLOC_FREE(tmp_ctx);