net: disable TestDNSThreadLimit even in non-short mode by default
net: disable TestDNSThreadLimit even in non-short mode by default

TestDNSThreadLimit creates tons of DNS queries and it occasionally
causes an unintentional traffic jam and/or crash of some virtual
machine software, especially its builtin networking stuff.

We can run TestDNSThreadLimit with -dnsflood flag instead.

LGTM=dave, rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/63600043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang