snapshot_par_cur: Add cases for missed option and negative testing

Add negative cases for invalid snapshotname and run command without
snapshot created first. Add positive case for '--name' option.

Signed-off-by: Wayne Sun <gsun@redhat.com>
snapshot_edit: Update snapshot_current return result

Using CmdResult according to the function change

Signed-off-by: Wayne Sun <gsun@redhat.com>