Test MDEV-26575 fails when it runs after MDEV-25389. This is because
the latter simulates a failure while an applier thread is
created in `start_wsrep_THD()`. The failure was not handled correctly
and would not cleanup the created THD from the global
`server_threads`. A subsequent shutdown would hang and eventually fail
trying to close this THD.
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>