Fix for Bug#21970.

The mysql-test-run-shell.sh script was improperly evaluating an expression for setting up multiple masters.

This fix adds the `expr $MASTER_MYPORT + 1` construct to the offending line in order for the expression to evaluate properly and produce 9307 instead of the string "9306+1"  

mysql-test/mysql-test-run-shell.sh

@@ -1793,7 +1793,7 @@ then
     $MYSQLADMIN --no-defaults --socket=$MASTER_MYSOCK1 -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
     $MYSQLADMIN --no-defaults --socket=$SLAVE_MYSOCK -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
     $MYSQLADMIN --no-defaults --host=$hostname --port=$MASTER_MYPORT -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
-    $MYSQLADMIN --no-defaults --host=$hostname --port=$MASTER_MYPORT+1 -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
+    $MYSQLADMIN --no-defaults --host=$hostname --port=`expr $MASTER_MYPORT + 1` -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
     $MYSQLADMIN --no-defaults --host=$hostname --port=$SLAVE_MYPORT -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
     $MYSQLADMIN --no-defaults --host=$hostname --port=`expr $SLAVE_MYPORT + 1` -u root -O connect_timeout=5 -O shutdown_timeout=20 shutdown > /dev/null 2>&1
     sleep_until_file_deleted 0 $MASTER_MYPID