Adds new parameter $restart_bindir for restart_mysqld.inc.

Example:

let $restart_bindir= /home/midenok/src/mariadb/10.3b/build;
--source include/restart_mysqld.inc

It is good to return back original server before check_mysqld will be
run at the test end:

let $restart_bindir=;
--source include/restart_mysqld.inc