Add tests to check for failing or missing service checks in consul
update.

Fixes a bug where an allocation is considered healthy if some of the
tasks are being restarted and as such, their checks aren't tracked by
consul agent client.

The underlying problem is that allocation registration in consul
agent/client code is mutable: tasks get removed as services from consul,
prior to stopping/restarting to allow for graceful removal from LBs.
The downside is that the health tracker may consider the allocation as
healthy if one of the task is down.

This uses the simplest approach to patch the problem by detecting the
number of expected checks against the registered checks.

I don't anticipate disrepency of counters.  `sreg.Checks` should only
contain checks that nomad agent explicitly registered and filter out
unexpected or unrelated checks:
https://github.com/hashicorp/nomad/blob/0ecda992317d3300e1c1da05170f8bba18410357/command/agent/consul/client.go#L1138-L1147
.

A better approach would have been to strictly compare the found check
IDs against an immutable list of expected IDs.  This sadly requires
significant code changes both to task runner service hooks and consul
hooks, that I'm not comfortable so close to cutting a new release.