Shutdown parsl executors in tests

[gpauloski]

Description

Shutdown parsl executors in tests, otherwise the dfk persists and pollutes later tests.

Fixes N/A

Type of Change

• Bug (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds or improves functionality)
• Internal (refactoring, performance, and testing)
• Breaking (fix or feature that would cause existing functionality to not work as expected)
• Documentation (no changes to the code)
• Development (CI workflows, packages, templates, etc.)
• Package (package dependencies and versions)

Testing

N/A

Pull Request Checklist

Please confirm the PR meets the following requirements.

• Relevant tags are added (breaking, bug, documentation, enhancement, package, etc.).
• Code changes pass pre-commit (e.g., ruff, mypy, etc.).
• Tests have been added to show the fix is effective or that the new feature works.
• New and existing unit tests pass locally with the changes.
• Docs have been updated and reviewed if relevant.