net: add test for lookupIPDeadline

Just to confirm the fix, by typing the follwing:
go test -run=TestLookupIPDeadline -dnsflood or
go test -run=TestLookupIPDeadline -dnsflood -tags netgo

Update issue 8602

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/166740043