BUG: Fix CLI running issues and add tests to prevent future regression.

Before, the CLI module logic would add the observer before running the CLI
and remove it once the CLI is modified. This proved to be error prone, for
example if two CLIs are ran at the same time. One CLI would finish and
remove the observation, preventing the other one from ever finishing.
Instead, we propose to always listen to the event. This make the code
simpler and more robust.

Thanks to Alexis and Dzenan for their help creating a simple test case.

Reported-by: Dzenan Zukic <dzenan.zukic@kitware.com>
Co-authored-by: Alexis Girault <alexis.girault@kitware.com>
Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Johan Andruejol <johan.andruejol@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24834 3bd1e089-480b-0410-8dfb-8563597acbee