Worker Manager: support test payloads in registerWorker

[petemoore]

For the purposes of integration testing, it is useful for worker tests to be able to call real taskcluster services in addition to mocks. This way, if the behavior of mocks and real services unexpectedly deviate, we can catch the issues before making a release.

Currently there is no way to successfully call workerManager.registerWorker for a non-static worker unless Worker Manager created the worker where the API call originates, which is not the case during integration testing. This is intentional in a production scenario, however severely limiting in an integration testing scenario.

In order to address this, it would be useful to reserve a special provisionerId (test-provisioner-id) that when used will slightly alter the behaviour of workerManager.registerWorker. Rather than looking up the worker in the database of Worker-Manager-spawned workers, and only returning successfully if exactly one suitable worker can be found, instead, the worker credentials, secret, and expiry can be given fake (reserved) values, and the worker config can be returned as if the call was successful.