discoverd/server: Retry UDP test query on timeout
discoverd/server: Retry UDP test query on timeout

The kernel provides no guarantees that a UDP packet will be
delivered, and this test occasionally flakes with:

    read udp 127.0.0.1:43870->127.0.0.1:43733: i/o timeout

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

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