Doctesting: don't test SageNB files unless SageNB has been installed #28834
Description
Since SageNB is now an optional package, we should detect whether it's present before doctesting its files via make ptestlong and similar commands.
Component: doctest framework
Author: John Palmieri
Branch/Commit: a579a9a
Reviewer: Frédéric Chapoton
Issue created by migration from https://trac.sagemath.org/ticket/28834