Support explicit expectation of no worker running

[0legg]

Right now there is no easy way to expect that some particular worker is not running during the given render step. There is a workaround to use requireExplicitWorkerExpectations, but it requires to specify the footprint for the rest of the workers.

Proposal: add methods like expectNoWorker/expectNoWorkerOutputting that will do the work complementary to expectWorker/expectWorkerOutputting by checking non-existence of a single worker with specific footprint.