Python: Added paragraph that warns about pytest.fixture(scope="session") with test server

[edmondop]

What does this PR do?

Addresses #3111 . When running tests in a project I noticed:

• Time skipping wasn't working even if it was enabled
• samples-python test never reuse the same workflow environment with timeskipping

This additional paragraph should warn developers familiar with pytest.fixtures not to use the scope session for caching the workflow environment