qa: fixing failures in TestShellOpts

We were seeing failures in TestShellOpts again
and again due the (newline added within the output)
output structure change with the cmd2 releases.
Now, we directly extract the final output of 'set editor'
command and use it, instead of traversing on complete output
through hardcoded indexes.This fix is better than earlier
hardcoded splitting/striping via indexes.

Fixes: https://tracker.ceph.com/issues/71795
Signed-off-by: Neeraj Pratap Singh <Neeraj.Pratap.Singh1@ibm.com>