image/gif: (temporarily) disable broken tests.

The compress/lzw encoder now rejects too-large input bytes, as of
https://go-review.googlesource.com/#/c/11227/, so we can't generate bad
GIFs programatically.

Change-Id: I0b32ce8e1f1776cd6997869db61e687430464e45
Reviewed-on: https://go-review.googlesource.com/11270Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/gif/reader_test.go

@@ -189,6 +189,12 @@ func TestBounds(t *testing.T) {
 }
 
 func TestNoPalette(t *testing.T) {
+	// https://go-review.googlesource.com/#/c/11227/
+	// changed the lzw encoder to reject input bytes that are too large,
+	// so that this test code no longer generates the right invalid GIF.
+	// TODO(nigeltao): re-enable this test somehow.
+	return
+
 	b := &bytes.Buffer{}
 
 	// Manufacture a GIF with no palette, so any pixel at all
@@ -215,6 +221,12 @@ func TestNoPalette(t *testing.T) {
 }
 
 func TestPixelOutsidePaletteRange(t *testing.T) {
+	// https://go-review.googlesource.com/#/c/11227/
+	// changed the lzw encoder to reject input bytes that are too large,
+	// so that this test code no longer generates the right invalid GIF.
+	// TODO(nigeltao): re-enable this test somehow.
+	return
+
 	for _, pval := range []byte{0, 1, 2, 3, 255} {
 		b := &bytes.Buffer{}