The problem was caused by a merge error (incorrect conflict resolution) when
the MDEV-7257 patch was merged into 10.1.

The incorrect merge put two code blocks in the wrong order. This caused a race
that was seen as sporadic test failures.

(The problem was that binlog end position was updated before running
after_flush hook; this way it was possible for the binlog dump thread to send
a transaction to a slave without requesting semi-sync acknowledgement. Then
when no acknowledgement was received, semisync replication would be disabled
on the master.)