dnsforward: imp test more

internal/dnsforward/dnsforward_test.go

@@ -583,19 +583,15 @@ func TestSafeSearch(t *testing.T) {
 		t.Run(tc.host, func(t *testing.T) {
 			req := createTestMessage(tc.host)
 
+			// TODO(a.garipov):  Create our own helper for this.
 			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
+			once := &sync.Once{}
+			require.EventuallyWithT(t, func(c *assert.CollectT) {
+				r, _, errExch := client.Exchange(req, addr)
+				if assert.NoError(c, errExch) {
+					once.Do(func() { reply = r })
+				}
 			}, testTimeout*10, testTimeout)
-			require.NoErrorf(t, err, "couldn't talk to server %s: %s", addr, err)
 
 			if tc.wantCNAME != "" {
 				require.Len(t, reply.Answer, 2)