tests: work with older dbus

On RHEL 5, with dbus 1.1.2, compilation failed with:

virsystemdmock.c: In function 'dbus_connection_send_with_reply_and_block':
virsystemdmock.c:68: warning: implicit declaration of function 'dbus_message_set_serial'

Fix this by instead bypassing all attempts to use a dbus serial.

* tests/virsystemdmock.c (dbus_message_set_reply_serial): Add new
override.
(dbus_connection_send_with_reply_and_block): No longer bother with
the serial.
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/virsystemdmock.c

@@ -58,6 +58,12 @@ dbus_bool_t dbus_connection_set_watch_functions(DBusConnection *connection ATTRI
     return 1;
 }
 
+dbus_bool_t dbus_message_set_reply_serial(DBusMessage *message ATTRIBUTE_UNUSED,
+                                          dbus_uint32_t serial ATTRIBUTE_UNUSED)
+{
+    return 1;
+}
+
 DBusMessage *dbus_connection_send_with_reply_and_block(DBusConnection *connection ATTRIBUTE_UNUSED,
                                                        DBusMessage *message,
                                                        int timeout_milliseconds ATTRIBUTE_UNUSED,
@@ -65,8 +71,6 @@ DBusMessage *dbus_connection_send_with_reply_and_block(DBusConnection *connectio
 {
     DBusMessage *reply = NULL;
 
-    dbus_message_set_serial(message, 7);
-
     if (getenv("FAIL_BAD_SERVICE"))
         reply = dbus_message_new_error(message,
                                        "org.freedesktop.systemd.badthing",