Remove _deserialize functions in worker run tests

These tests https://gitlab.com/teklia/arkindex/backend/-/blob/master/arkindex/process/tests/test_workerruns.py have a bunch of deserializing functions, and while working on https://gitlab.com/teklia/arkindex/backend/-/issues/1367 I realized how annoying they were, as they prevent you from seeing what's actually happening / being returned before the deserialization.