improving a test that shows a failure.

the wait condition was for data in tables but the 
log positions are updates after the data are unlocked.
So there was a time window
[after_table_unlock_for_select, log_pos_updated] where the
orig cond was true but log position might be changed.
the correct one is to expect the last pos of the
slave's insert in the output of show_slave_status on the
master.


mysql-test/suite/rpl/t/rpl_flushlog_loop.test:
  refining a sychronization condition (2 times)