1. sync_binlog=1 is needed to ensure that on flush binlog is available on
   disk.
2. Insert on 'connection master2' should wait for 'master1_ready' from
   'connection master1'.

Added sync_binlog=1 to all tests that got added as part of MDEV-21117 fix

@mysql-test/suite/binlog/t/binlog_truncate_multi_log_unsafe.test
@mysql-test/suite/binlog/t/binlog_truncate_multi_engine.test
@mysql-test/suite/binlog/t/binlog_truncate_active_log.test
@mysql-test/suite/binlog/t/binlog_truncate_multi_log.test