Worker.Tests carried its own hand-rolled FakeWorkerClient implementing the
whole IWorkerClient surface, duplicating StubWorkerClient in Ui.Tests. Every
hub-signature change had to be applied twice. Link StubWorkerClient.cs into
Worker.Tests via a csproj Compile Include and subclass it there; add
SetTaskStatusCalls tracking to the shared stub (the one behavior Worker.Tests
needed that Ui.Tests didn't have).