conftest: Refactor GraphServerProcess to be more flexible
- queue is now encapsulated
- object in the queue is now a dict instead of a tuple (to allow adding more keys easily in the future + for readability)
This will be used in a future commit (which will address swh/infra/sysadm-environment#4497 (closed)), to get the queue's object twice from different functions.
Migrated from D8402 (view on Phabricator)