Fix test_pelican.py to ensure any directory while starting pytest.

[egberts]

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

Issue

When within pelican/pelican/tests subdirectory, following tests failed in PyCharm IDE:

• test_main_on_content()
• test_main_on_content_markdown_disabled()

Also due to missing pelican Python file, this test fails.

Only way to pass these tests is to change working directory to top of the Pelican package (root dir).

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