test-sync: implement send_reply()

test-sync: implement send_reply()

Implement a new helper function to reply to pending messages.

Formerly, the test abused the timeout parameter to respond, knowing
that its value will eventually end up in the kdbus message in the
same union as the cookie_reply field.

In the process of switching to absolute timeouts, however, this bites
us, so move this hack out of the way first.

Signed-off-by: Daniel Mack <daniel@zonque.org>