[cicd] add pytest markers for cuda and slow tests

[Moenupa]

What does this PR do?

Refactors tests using pytest.mark for unit testing.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?