test: move snap delete operation to non-root directory

test: move snap delete operation to non-root directory

In teuthology, root directory in a Ceph Filesystem is owned
by root (0:0) and has sticky-bit enabled. Snapshots created
on the directory carry its UID/GID and mode.

This causes tests that remove snapshots on root directory to
fail. Sticky bit on a directory gives permission to delete a
file in that directory only if you own the file.

Signed-off-by: Venky Shankar <vshankar@redhat.com>