net/http: deflake TestClientTimeout

This test was only enabled by default today so it hasn't been hardened
by build.golang.org. Welcome to the ring, TestClientTimeout.

Change-Id: I1967f6c825699f13f6c659dc14d3c3c22b965272
Reviewed-on: https://go-review.googlesource.com/33101
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/client_test.go

@@ -1200,9 +1200,9 @@ func testClientTimeout(t *testing.T, h2 bool) {
 			return
 		}
 		if r.URL.Path == "/slow" {
+			sawSlow <- true
 			w.Write([]byte("Hello"))
 			w.(Flusher).Flush()
-			sawSlow <- true
 			select {
 			case <-testDone:
 			case <-time.After(timeout * 10):