mtr galera_3nodes.galera_garbd_backup test failures

As reported in zulip chat: https://mariadb.zulipchat.com/#narrow/channel/236699-Buildbot/topic/MDBF-976.20.2B.20galera.20.2B.20mem

1) isn't doing anything to gain a reliable innodb state on node1 before it does its first checksum
2) there's assumptions that the order of the files in the find ... -exec md5sum appear consistently in the same order.
And before final checksum calculation of after:

SET GLOBAL innodb_max_dirty_pages_pct=0;
Should have a wait condition like in 93522bc9a987e756ae1872953af1ddc1731ce9b6 that ensures the dirty pages are 0 before the next checksum.