SetDERPMap spawns a goroutine that calls ReSTUN, which logs via the test logger. If the test returns before that goroutine logs, the goroutine races with testing cleanup. Use tstest.WhileTestRunningLogger so the goroutine's logf call becomes a no-op once the test finishes. Fixes #19829 Change-Id: I1097f98e40ffd1c5dd7fb7a715c918255853e3c6 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>