MDEV-26210: Disable mariabackup.log_page_corruption

MDEV-26193 exposed a race condition in the test
mariabackup.log_page_corruption by no longer waking up purge tasks
on every transaction commit. (Note that there never was a guarantee
that the purge tasks would actually run as quickly as the test expected
it to happen; that would depend on the CPU load.)

mysql-test/suite/mariabackup/disabled.def

+log_page_corruption : MDEV-26210