Bug #31240 load data infile replication between (4.0 or 4.1) and 5.1 fails

      
Overriding a default value of --secure-file-priv on pb.mtr to be $MYSQLTEST_VARDIR which 
makes pb's mtr happy to execute the regression test for the bug.

mysql-test/suite/rpl/t/rpl_cross_version-master.opt

---replicate-same-server-id --relay-log=slave-relay-bin --secure-file-priv=$MYSQL_TEST_DIR
+--replicate-same-server-id --relay-log=slave-relay-bin --secure-file-priv=$MYSQLTEST_VARDIR