Bug#20588: mysqldump.test may fail, depending on system-wide configuration

mysqldump.test calls my_print_defaults in a way that includes the systemwide
my.cnf, so the results will be beyond our control and depend on whatever the
user has in their my.cnf, namely the [mysqldump] section.

call my_print_defaults with --config-file rather than --defaults-extra-file
to prevent inclusion of system-wide defaults and use our config-file only.

mysql-test/t/mysqldump.test

@@ -610,7 +610,7 @@ drop database db1;
 # BUG#15328 Segmentation fault occured if my.cnf is invalid for escape sequence
 #
 
---exec $MYSQL_MY_PRINT_DEFAULTS --defaults-extra-file=$MYSQL_TEST_DIR/std_data/bug15328.cnf mysqldump
+--exec $MYSQL_MY_PRINT_DEFAULTS --config-file=$MYSQL_TEST_DIR/std_data/bug15328.cnf mysqldump
 
 
 #