net/http: wait longer for subprocess to startup in test

Might deflake the occasional linux-amd64-race failures.

Change-Id: I273b0e32bb92236168eb99887b166e079799c1f1
Reviewed-on: https://go-review.googlesource.com/22858
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/serve_test.go

@@ -4268,7 +4268,7 @@ func BenchmarkClient(b *testing.B) {
 	// Wait for the server process to respond.
 	url := "http://localhost:" + port + "/"
 	for i := 0; i < 100; i++ {
-		time.Sleep(50 * time.Millisecond)
+		time.Sleep(100 * time.Millisecond)
 		if _, err := getNoBody(url); err == nil {
 			break
 		}