Given a list of HealthChecks, this determines the "best" status for the
collective group. This is useful for nodes and services, which may have
multiple checks associated with them.
This is necessary as consul-api's tests require a real consul instance
to be running. We can't directly import an agent to fire up an instance,
due to the way this would create an import cycle. These tests instead
will start a consul instance using the binary in $PATH (if it exists).
