qa: create a new directory to fill the volume space

qa: create a new directory to fill the volume space

When trying to filling the volume space by continuing filling multiple
files, and when flushing the dirty caps back to MDS the MDS will try
to skip updating the parent rstat in 'mds_dirstat_min_interval' to
avoid propagating more often than this. That means the quota changes
couldn't be broadcasted to the clients in time.

So after waiting for 20 seconds, and if we try to write the existing
files only the first file could successfully update the parent quota
realm in MDS, but this won't increase the total size.

Fixes: https://tracker.ceph.com/issues/59349
Signed-off-by: Xiubo Li <xiubli@redhat.com>