Fix flaky rpl.binlog_trx_meta_data test

Summary:
Fix the race between `flush logs` and `purge logs` by syncing the slave
with the master before purging.

Test Plan: MTR

Reviewers: herman, yashb, arahut

Reviewed By: yashb

Subscribers: [email protected]

Differential Revision: https://phabricator.intern.facebook.com/D10247529

Tasks: T34877553

Signature: 10247529:1539106810:2596e420b78ebf76610c2521aa60477528c3d6e8

mysql-test/suite/rpl/include/binlog_trx_meta_data.inc

@@ -1,8 +1,8 @@
 # Init
 connection master;
+flush logs;
 sync_slave_with_master;
 connection master;
-flush logs;
 let $master_last_log = query_get_value("SHOW MASTER STATUS", "File", 1);
 eval purge binary logs to '$master_last_log';
 connection slave;