connection reuse in workflow tests

[soxofaan]

While working on #37 I noticed that there is almost a full openeo.connect + auth + capabilities discovery cycle for each workflow test, which is quite time consuming, especially if you are using process/level filtering features to skip certain tests.

Reason is that most workflow tests are isolated in their own module and the connection fixture has "module" scope, meaning that a new connection is created (auth+capability discovery included) for each module.

@clausmichele I think we should consider grouping workflow tests more, e.g. per level or something.

Note: this is just a "nice to have" task, I don't think this some kind of blocker for closure of the current project. I just wanted to park that idea here