rpl_semi_sync_no_missed_ack_after_add_slave fails after MDEV-35109

MTR test rpl_semi_sync_no_missed_ack_after_add_slave fails on buildbot after the preparatory commit for MDEV-35109 (5290fa043b1) which changed a sleep to a debug_sync point. The problem is that the debug_sync point would time-out on a slave while waiting to enter the logic to send an ACK reply. More specifically, where the test config is a primary with two replicas, and the test waits on one of the replicas to start sending an ACK, if the other replica was able to receive the event and respond with an ACK before the binlog dump thread of the timing-out server would prepare to send event, it wouldn't set the SEMI_SYNC_NEED_ACK flag, and the replica wouldn't even try to respond with an ACK.