Fixes rpl_stm_loaddata_concurrent failure in PB2.

The test case did not start with fresh binlogs, so in some
cases, dependending on the order MTR runs the tests, it would
try to show binlog contents from invalid positions (binary log
would contain unexpected events from previous test).

We fix this by deploying a RESET MASTER at the beginning of the
test case.

mysql-test/suite/rpl/r/rpl_stm_loaddata_concurrent.result

+RESET MASTER;
 CREATE TABLE t1 (c1 char(50));
 LOAD DATA INFILE '../../std_data/words.dat' INTO TABLE t1;
 LOAD DATA CONCURRENT INFILE '../../std_data/words.dat' INTO TABLE t1;

mysql-test/suite/rpl/t/rpl_stm_loaddata_concurrent.test

 -- source include/not_ndb_default.inc
 -- source include/have_log_bin.inc
 -- source include/have_binlog_format_statement.inc
+RESET MASTER;
 
 let $binlog_start= query_get_value(SHOW MASTER STATUS, Position, 1);
 CREATE TABLE t1 (c1 char(50));