Add `SetAsDispatched` outbox persistence tests

[timbussmann]

Describe the suggested improvement

Is your improvement related to a problem? Please describe.

The persistence tests around the outbox storage option are limited and currently don't have test cases that verify the expected behavior of IOutboxStorage.SetAsDispatched.

Therefore, it seems different persisters also developed different behavior for certain edge cases, e.g. when dealing with concurrency (some throw exceptions, some don't - the correct behavior is probably to overwrite the entry and not throw)

Describe the suggested solution

Add a set of test cases that can be shared with persistence downstreams to verify implementations behave as expected and consistently.

Additional Context

Changes to persistence tests should always be verified on all persistence implementations before shipping them to ensure that updating the test dependencies on the downstream doesn't cause unexpected problems.