When a file is marked # sage.doctest: optional - xyz, we omit it from doctesting unless --optional=xyz is given.

This will save us from having to add lots of # optional - ... tags to files in the course of modularization (#29705)

We do this by extending sage.doctest.control.skipfile, which already parses files for # nodoctest file directives.

Previous related proposals/discussions: #3260, #20427

Also related: #30746

CC: @simon-king-jena @kiwifb @roed314 @saraedum @nthiery @videlec

Component: doctest framework

Keywords: sd111

Author: Matthias Koeppe, John Palmieri

Branch/Commit: 5cc1288

Reviewer: John Palmieri, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30778