Fixes for test collection and guards for missing PhysicsNeMo-Sym

[peterdsharpe]

PhysicsNeMo Pull Request

Description

Addresses two bugs:

• Previously, running pytest . from root caused an error in test collection due to path issues in test_runner.py. Fixed this, and refactored to use the more-modern pathlib builtin library instead of os.path. (In general, going forward, we should typically prefer pathlib over os.path)
• There was (and still is) a bug that one of the docs-folder test scripts in the PhysicsNeMo repo depends on PhysicsNeMo-Sym. This fails on a vanilla PhysicsNeMo install, as Sym is not included. My current understanding is that the PhysicsNeMo -> PhysicsNeMo Sym dependency was intended to be only one-way: sym depends on core, but core does not depend on sym.

This provides a fix for the first bug, and at least a more graceful failure for the second bug.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The CHANGELOG.md is up to date with these changes.
• An issue is linked to this pull request.

Dependencies

[peterdsharpe]

/blossom-ci