discoverd/server: Don't assert that DNS server has closed in test suite
discoverd/server: Don't assert that DNS server has closed in test suite

Sometimes it fails on Go 1.5 if it doesn't shutdown fast enough, this
shouldn't be an issue because we use new ports for every server.

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

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