mysqldump: racing condition in STDERR+STDOUT output

There can be test failure when mysqldump is executed like this:

--exec $MYSQL_DUMP ... 2>&1

STDERR is joined to STDOUT while STDOUT buffer is in undetermined state.

Fix

Call fflush(md_result_file) before each fprintf(stderr, ...). Better ensure that md_result_file is STDOUT, otherwise flush is not needed.