Reduce EventGrid publishing duplication in message handler test fixtures

[stijnmoreels]

Is your feature request related to a problem? Please describe.
We have quite some duplication on some of the message handlers used during integration testing. There is an extension on the EventGrid publisher to reduce this duplication PublishOrderAsync.

Describe the solution you'd like
Use the PublishOrderAsync extension to publish events in test fixture message handlers. This will also help us move away easier from the IEventGridPublisher in a later phase.