Add more test case handling of PATH, et al variables inside pelicanconf.py

[egberts]

• I have read the Filing Issues and subsequent “How to Get Help” sections of the documentation.
• I have searched the issues (including closed ones) and believe that this is not a duplicate.

Issue

This issue is broken off of issue #3357 due to entirely different scoping effort of the following functions within settings.py/test_settings.py:

• load_source() addressed by Many new unit tests for configuration settings file (pelicanconf.py) (#3357) #3368 and
• configure_settings() that this issue shall focus on.

This issue deals with PATH, et al., variable name in a Pelican configuration settings (pelicanconf.py) file.

Relative Outside Subdirectory does not exist

Where the PATH is defined to 'content' and that (relative-type) 'content' subdirectory does NOT exist.

$ python -m pelican -D -s /tmp/pelicanconf.py

    CRITICAL Exception: `PATH` object in /tmp/pelicanconf.py is undefined. You need __init__.py:696
        to specify a path containing the content (see pelican --help for more                 
        information)     

An ERROR should have said: content subdirectory (as defined in 'PATH' variable) does not exist.

Undefined PATH

Where the PATH variable is undefined in pelicanconf.py

$ python -m pelican -s /tmp/pelicanconf.py -D content

       CRITICAL Exception: `PATH` object in /tmp/pelicanconf.py is undefined. You need __init__.py:696
                to specify a path containing the content (see pelican --help for more                 
                information)  

A WARNING should have said: 'PATH' variable is undefined; using 'content' as a default.

Platform

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Platform

• OS version and name: Linux 6.1.0-21-amd64 SMP PREEMPT_DYNAMIC Debian 6.1.90-1 (2024-05-03) x86_64 GNU/Linux
• Python version: 3.11.2
• Pelican version: HEAD (513abbf)
• Link to theme: m.css
• Links to plugins: pelican-plugins
• Link to your site: n/a
• Link to your source: n/a