selenium: invoke jobs to enforce eventual consistency

Many tests use the API during their setup phase, which does not
participate in clearing the job queue, but constantly adds new items.
Consequently the job count keeps mounting. In order to be able to assert
the state of the application once all jobs are processed we need a way
to invoke them.
This is a minimalistic alternative to Idd4924e16925cbf0cb9e42645670c3fd76b72353

Bug: T199446
Change-Id: Ifc49bc5c06eef71da8aff1b1837c32debc2c45dd