net/http: simplify ctx.Err() call in test

See CL 40291. ctx.Err() is defined to only return non-nil exactly
when ctx.Done() returns a closed channel.

Change-Id: I12f51d8c42228f759273319b3ccc28012cb9fc73
Reviewed-on: https://go-review.googlesource.com/71310
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -686,12 +686,8 @@ func TestHTTP2WriteDeadlineExtendedOnNewRequest(t *testing.T) {
 		req = req.WithContext(ctx)
 
 		r, err := c.Do(req)
-		select {
-		case <-ctx.Done():
-			if ctx.Err() == context.DeadlineExceeded {
-				t.Fatalf("http2 Get #%d response timed out", i)
-			}
-		default:
+		if ctx.Err() == context.DeadlineExceeded {
+			t.Fatalf("http2 Get #%d response timed out", i)
 		}
 		if err != nil {
 			t.Fatalf("http2 Get #%d: %v", i, err)