Fix that MTR leaks restart: options to following test

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.

mysql-test/mysql-test-run.pl

@@ -5215,6 +5215,7 @@ sub server_need_restart {
     if (!My::Options::same($started_opts, $extra_opts) ||
         exists $server->{'restart_opts'})
     {
+      delete $server->{'restart_opts'};
       my $use_dynamic_option_switch= 0;
       if (!$use_dynamic_option_switch)
       {