Use single job to send events (so they are not sent in parallel in
random order).

Since the events are now sent async, we need wait for jobs before we can
compare the expected target.

Change-Id: I57dd5a991d0646ca123c90cf93751fd0893afb74
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/190867
Reviewed-by: Jörg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>
Tested-by: PDE Bot <pde-bot@eclipse.org>