Tests: disable `accelerate_tests` mark warnings
# What does this PR do?

Adds the `accelerate_tests` mark to `conftest.py`, so we don't get related warnings at test time. 

Here's a print screen before the fix:
<img width="1512" alt="Screenshot 2023-04-05 at 11 27 32" src="https://user-images.githubusercontent.com/12240844/230054803-cc95b93d-aab4-4133-baa8-7115760cd3ee.png">

And after the fix:
<img width="1512" alt="Screenshot 2023-04-05 at 11 27 53" src="https://user-images.githubusercontent.com/12240844/230054894-46a9435a-43be-4ef9-b111-92d664017d13.png">

_The documentation is not available anymore as the PR was closed or merged._