test/librados_test_stub: unbreak selfmanaged_snap_rollback()

test/librados_test_stub: unbreak selfmanaged_snap_rollback()

After the rollback assert in TestGroup.add_snapshot{,PP} was made
meaningful in the previous commit, it fails in mock tests which means
that rollback has never been exercised properly...

While I confess to not following file->snap_id == CEPH_NOSNAP branch
especially given how file variable is shadowed, it's pretty clear that
get_snap_read() doesn't belong here -- the snapshot selected for reads
has nothing to do with rollback.  Replacing it with the rollback snap
ID makes sense of the other branches and makes the tests in question
pass.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>