The failing test case validates Seconds_Behind_Master for a delayed
slave, while STOP SLAVE is executed during a delay. The test fixes
initially added to the test (commit b04c8575) added a table lock
to ensure a transaction could not finish before validating the
Seconds_Behind_Master field after SLAVE START, but did not address a
possibility that the transaction could finish before running the
STOP SLAVE command, which invalidates the validations for the rest
of the test case. Specifically, this would result in 1) a timeout in
“Waiting for table metadata lock” on the replica, which expects the
transaction to retry after slave restart and hit a lock conflict on
the locked tables (added in b04c8575), and 2) that
Seconds_Behind_Master should have increased, but did not.

The failure can be reproduced by synchronizing the slave to the master
before the MDEV-32265 echo statement (i.e. before the SLAVE STOP).

This patch fixes the test by adding a mechanism to use DEBUG_SYNC to
synchronize a MASTER_DELAY, rather than continually increase the
duration of the delay each time the test fails on buildbot. This is
to ensure that on slow machines, a delay does not pass before the
test gets a chance to validate results. Additionally, it decreases
overall test time because the test can continue immediately after
validation, thereby bypassing the remainder of a full delay for each
transaction.