online tests: use gitlab for auth failures

GitHub recently changed their behavior from returning 401s for private
or nonexistent repositories on a clone to returning 404s.  For our tests
that require an auth failure (and 401), move to GitLab to request a
missing repository.  This lets us continue to test our auth failure
case, at least until they decide to mimic that decision.

tests/online/clone.c

@@ -334,7 +334,7 @@ void test_online_clone__cred_callback_called_again_on_auth_failure(void)
 	git__free(_remote_url);
 	git__free(_remote_user);
 
-	_remote_url = git__strdup("https://github.com/libgit2/non-existent");
+	_remote_url = git__strdup("https://gitlab.com/libgit2/non-existent");
 	_remote_user = git__strdup("libgit2test");
 
 	g_options.fetch_opts.callbacks.credentials = cred_count_calls_cb;