TT#111150 Fix memory leak in test

Change-Id: I8461441516cf04f5da811ca5cdfca5a4029d7bf2
Warned-by: gcc ASAN

t/test-dtmf-detect.c

@@ -646,6 +646,7 @@ int main(int argc, char **argv) {
 				"code 0 level -99 delay 816, "))
 		abort();
 
+	g_string_free(output, TRUE);
 	dtmf_rx_free(dtmf_dsp);
 
 	return 0;