net: increase TestNotTemporaryRead server sleep

On aix/ppc64, if the server closes before the client calls Accept,
this test will fail.

Increasing the time before the server closes should resolve this
timeout.

Updates #29685

Change-Id: Iebb849d694fc9c37cf216ce1f0b8741249b98016
Reviewed-on: https://go-review.googlesource.com/c/158038Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/net_test.go

@@ -529,7 +529,7 @@ func TestNotTemporaryRead(t *testing.T) {
 	server := func(cs *TCPConn) error {
 		cs.SetLinger(0)
 		// Give the client time to get stuck in a Read.
-		time.Sleep(20 * time.Millisecond)
+		time.Sleep(50 * time.Millisecond)
 		cs.Close()
 		return nil
 	}