This was found during MDEV-12179 development, but unrelated so put in its
own commit.

The "restart: --extra-option" facility in expect file for server restart
sets extra options for the restarted mysqld. These options were incorrectly
not cleared when starting a new test case, so the server was restarted with
whatever extra options were left by the previous testcase, causing random
failures depending on order of running tests.