The test mariabackup.mdev-14447 is inserting relatively much
data while concurrently backing up the data. The test often fails
on CI systems, possibly due to an inherent race condition between
the producer (server) and consumer (backup) that would be solved
if the backup was being produced by the server (MDEV-14992).

The written data volume was increased somewhat by
commit 4179f93d (MDEV-18976).
Let us trim the log volume by not writing PAGE_CHECKSUM records
or row-level undo log records, and make the test cover what was
intended to cover by creating the table in the system tablespace.