net/http: deflake TestResponseWriterWriteStringAllocs, test interface instead

Skip the allocation testing (which was only used as a signal for
whether the interface was implemented by ResponseWriter), and just
test for it directly.

Fixes #9575

Change-Id: Ie230f1d21b104537d5647e9c900a81509d692469
Reviewed-on: https://go-review.googlesource.com/2720Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/net/http/serve_test.go

@@ -2384,18 +2384,24 @@ func TestRequestBodyCloseDoesntBlock(t *testing.T) {
 	}
 }
 
-func TestResponseWriterWriteStringAllocs(t *testing.T) {
+// test that ResponseWriter implements io.stringWriter.
+func TestResponseWriterWriteString(t *testing.T) {
+	okc := make(chan bool, 1)
 	ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
-		if r.URL.Path == "/s" {
-			io.WriteString(w, "Hello world")
-		} else {
-			w.Write([]byte("Hello world"))
+		type stringWriter interface {
+			WriteString(s string) (n int, err error)
 		}
+		_, ok := w.(stringWriter)
+		okc <- ok
 	}))
-	before := testing.AllocsPerRun(50, func() { ht.rawResponse("GET / HTTP/1.0") })
-	after := testing.AllocsPerRun(50, func() { ht.rawResponse("GET /s HTTP/1.0") })
-	if int(after) >= int(before) {
-		t.Errorf("WriteString allocs of %v >= Write allocs of %v", after, before)
+	ht.rawResponse("GET / HTTP/1.0")
+	select {
+	case ok := <-okc:
+		if !ok {
+			t.Error("ResponseWriter did not implement io.stringWriter")
+		}
+	default:
+		t.Error("handler was never called")
 	}
 }