Several SharedFuture tests add multiple continuations to the shared future while a background thread emplaces into the linked promise. This produced a race condition where the background thread might have to schedule() work for a continuation that's already been registered at the same time as the test thread is registering a new continuation. The InlineQueuedCountingExecutor, however, is not thread safe, and concurrent calls to schedule() on it can result in a data race and thus a test failure. We should switch these tests to use the InlineRecursiveCountingExecutor instead, which is thread safe.