test: say that nilptr2.go requires 256+MB stack

Fixes #6161.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13059043

test/nilptr2.go

@@ -87,8 +87,8 @@ var tests = []struct{
 	{"&bigstructp.x[i]", func() { println(&bigstructp.x[i]) }},
 	{"bigstructp.x[9]", func() { println(bigstructp.x[9]) }},
 	{"&bigstructp.x[9]", func() { println(&bigstructp.x[9]) }},
-	{"bigstructp.x[200<<20]", func() { println(bigstructp.x[200<<20]) }},
-	{"&bigstructp.x[200<<20]", func() { println(&bigstructp.x[200<<20]) }},
+	{"bigstructp.x[100<<20]", func() { println(bigstructp.x[100<<20]) }},
+	{"&bigstructp.x[100<<20]", func() { println(&bigstructp.x[100<<20]) }},
 	{"bigstructp.l", func() { println(bigstructp.l) }},
 	{"&bigstructp.l", func() { println(&bigstructp.l) }},
 	{"m1.F()", func() { println(m1.F()) }},
@@ -109,7 +109,7 @@ type BigStruct struct {
 	i int
 	j float64
 	k string
-	x [256<<20]byte
+	x [128<<20]byte
 	l []byte
 }