Add tests for Deadman's switch

Adding tests is not feasible until the deadman's switch has its own clock. See #691

While the intent of the deadman's switch it to be independent of the normal task clock, that doesn't mean it can't have its own clock. In the context of both replays and tests it makes sense.

Start by reading issue #691 and the deadman's switch implementation to understand the proposed independent clock. Determine the clock's required behavior in replays and tests, then add coverage once that prerequisite exists. Done means the deadman's switch has an isolated clock and tests exercise its timing independently of the normal task clock.

Written by the indexing model from the issue text.