test: avoid using cmdport equal to ntpport in system tests

Make sure the two randomly generated port numbers used in system tests
are different to avoid failures.

test/system/test.common

@@ -213,7 +213,10 @@ generate_chrony_conf() {
 
 	user=$(get_user)
 	ntpport=$(get_free_port)
+	while true; do
 		cmdport=$(get_free_port)
+		[ "$ntpport" -ne "$cmdport" ] && break
+	done
 
 	echo "0.0 10000" > "$TEST_LIBDIR/driftfile"
 	echo "1 MD5 abcdefghijklmnopq" > "$TEST_DIR/keys"