spider/bugfix.mdev_29904 fails with "Server shutdown in progress" in CI

E.g. https://buildbot.mariadb.net/buildbot/builders/kvm-asan/builds/10208/steps/mtr_nm/logs/stdio

I am not sure why this happens and I cannot reproduce it at the same commit. Here is the complete test:

--echo #

--echo # MDEV-29904 SPIDER plugin initialization fails upon startup

--echo #

--let $restart_parameters=--plugin-load-add=ha_spider

--source include/restart_mysqld.inc

According to [1] the error message means some query is still running while the server is shutting down. Maybe mtr tries to shutdown the server after --source include/restart_mysqld.inc while the spider init queries are still running in another thread? In that case perhaps we need to introduce a wait at the end of the test

[1] https://www.percona.com/blog/error-mysqld-sort-aborted-server-shutdown-in-progress/

To add a wait condition perhaps we could use include/wait_condition.inc, but that would require a sql statement to determine the completion, but looking at at spd_init_query.h, I don't see which statement can achieve this, so maybe we'll need to add an init query that signals completion.

On the other hand, if spider init queries are no longer running in a background thread, then this bug may be fixed automatically, so let's block this ticket with the init bug ticket MDEV-22979 where we are considering such change.