net: remove redundant test case for lookupIP with threadLimit
There is no reason to have the redundant test case TestDNSThreadLimt
because TestLookupIPDeadline does cover what we need to test with
-dnsflood flag and more.
Also this CL moves TestLookupIPDeadline into lookup_test.go to avoid
abusing to control the order of test case execution by using file name.
Change-Id: Ib417d7d3411c59d9352c03c996704d584368dc62
Reviewed-on: https://go-review.googlesource.com/2204 Reviewed-by: Ian Lance Taylor <iant@golang.org>