Document test_sessions

[szokeasaurusrex]

test_sessions.py is a bit confusing. This was brought up in a recent code review. We should add some comments to clarify what we are trying to test.

Tests are always also a kind of documentation on how to use the code.

Could you add comments to the three sessions to mark what aggregate status is expected for each one of them?

Please add those comments to all similar tests in this file. This will help us in the future (or others) to understand what is going on.

Originally posted by @antonpirker in #3419 (comment)