discoverd/health: Fix logical race in monitor test
discoverd/health: Fix logical race in monitor test

Sometimes the ticker will be selected instead of the channel close,
resulting in a double send to done.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

lmars/flynn,philiplb/flynn,lmars/flynn,flynn/flynn,lmars/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,flynn/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,philiplb/flynn,philiplb/flynn,flynn/flynn,benbjohnson/flynn,benbjohnson/flynn,benbjohnson/flynn,benbjohnson/flynn