qa: Fix test_subvolume_inherited_snapshot_info

The follwoing previous test creates 25 subvolumes
but doesn't wait for trash to be empty.

test_root_snapshot_with_use_global_snaprealm_seq_config_disabled_with_journal_flush

While the mgr/volumes purge thread is waiting for the unlink reply
from mds, the next test's setup cleans up the filesystem
and restarts mds. This causes the purge thread to hang and
doesn't purge entries from _deleting.

Fixes: https://tracker.ceph.com/issues/78208
Signed-off-by: Kotresh HR <khiremat@redhat.com>