net: TestSpecialDomainName can hang indefinitely

[mundaym]

The DNS test TestSpecialDomainName can hang indefinitely because it sends a DNS request using UDP and waits for a response without specifying a timeout.

The easiest fix would be to add a timeout to the call to exchange so that the test fails quickly with a nice error message if the request gets lost or is ignored. For example:

msg, err := exchange(server, tt.name, tt.qtype, 3 * time.Second)

[bradfitz]

You want to send a change, or do you want us to?

[mundaym]

I'm happy to send a change.

[mundaym]

CL sent https://go-review.googlesource.com/18661

[gopherbot]

CL https://golang.org/cl/18661 mentions this issue.

[danp]

Ref #13295

Might be nice to not involve the network for these at some point.