Buffer ordering in testing pipeline

[mat-hek]

The standard way to validate buffers in integration tests is to end the pipeline with the testing sink and do assertions like assert_sink_buffer(pipeline, sink, %Buffer{...}). However, making multiple assertions like that doesn't guarantee that buffers come in in the order of those assertions. I suppose we should provide a mechanism to retrieve all the needed buffers/events/notifications as a list to be able to assert on whatever feature needed.

What is more, to make sure no other buffer came apart from the asserted ones, refute_sink_buffer(pipeline, sink, _) has to be used. This, however, waits for two seconds by default, making tests execute longer.

[mat-hek]

Moved from #239, see the full discussion there