MDEV-29934 rpl.rpl_start_alter_chain_basic, rpl.rpl_start_alter_restart_slave sometimes fail in BB with result content mismatch

rpl.rpl_start_alter_chain_basic was used to fail sporadically due
to a missed GTID master-slave synchronization which was necessary
because of the following SELECT from GTID-state table.

Fixed with arranging two synchronization pieces for two
chain slaves requiring that.

Note rpl.rpl_start_alter_restart_slave must have been fixed by
MDEV-30460 and 87e13722 (manual) merge commit.