net/dns/resolver: fix data race in test

Fixes #17339

Change-Id: I486d2a0e0931d701923c1e0f8efbda99510ab19b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f