Require ruff and isort on the test suite

[keileg]

Proposed changes

This PR requires that files in the tests folder are formatted with ruff and isort for the static GH action check to pass.

We do not do require mypy on the tests as this is highly unlikely to give a positive cost-benefit ratio, at least for now.

Types of changes

What types of changes does this PR introduce to PorePy?
Put an x in the boxes that apply.

• Minor change (e.g., dependency bumps, broken links).
• Bugfix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [] Testing (contribution related to testing of existing or new functionality).
• Documentation (contribution related to adding, improving, or fixing documentation).
• Maintenance (e.g., improve logic and performance, remove obsolete code).
• Other: GH actions

Checklist

Put an x in the boxes that apply or explain briefly why the box is not relevant.

• The documentation is up-to-date.
• Static typing is included in the update.
• This PR does not duplicate existing functionality.
• The update is covered by the test suite (including tests added in the PR).
• If new skipped tests have been introduced in this PR, pytest was run with the --run-skipped flag.

[IvarStefansson]

This will make working with tests epsilon more agreeable for all of us!