net: avoid race on test hooks with DNS goroutines

net: avoid race on test hooks with DNS goroutines

The DNS code can start goroutines and not wait for them to complete.
This does no harm, but in tests this can cause a race condition with
the test hooks that are installed and unintalled around the tests.
Add a WaitGroup that tests of DNS can use to avoid the race.

Fixes #21090

Change-Id: I6c1443a9c2378e8b89d0ab1d6390c0e3e726b0ce
Reviewed-on: https://go-review.googlesource.com/82795
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>