One simple test for recovery after a single "crash" + one test of handling "infinitely" crashing kernels, i.e. that the restart limit is respected.

This is mainly due to the weakness of the previous method: `is_alive` only tests that the kernel process is alive, it does not indicate that the kernel has successfully completed startup. To solve this correctly, we would need to wait for a kernel info reply, but it is not necessarily appropriate to start a kernel client + channels in the restarter. Therefore, we use a "has been alive continuously for X time" as a heuristic for a stable start up.