dnsforward: fix test

internal/dnsforward/dnsforward_test.go

@@ -584,7 +584,13 @@ func TestSafeSearch(t *testing.T) {
 			req := createTestMessage(tc.host)
 
 			var reply *dns.Msg
+
+			// Use a mutex, since require.Eventually uses goroutines.
+			mu := &sync.Mutex{}
 			require.Eventually(t, func() (ok bool) {
+				mu.Lock()
+				defer mu.Unlock()
+
 				reply, _, err = client.Exchange(req, addr)
 
 				return err == nil