Factor out reusable code in BaseAggregator tests

And free ressources
Start handling EOS + test it
Add a linear pipeline test
By wignal we meant broadcast
We do not need the 'first' property anymore thanks to cookie sync
Avoid a race when stopping while not waiting for aggregate
Return the result of ->aggregate in chain as our flow return
Add a simple multi src test
Make sections and rename gstelement vmethods impl
Start implementing seeking support + test it
aggregator: add a default query function