go/ast: use single-element map in test

Avoids test failure due to undefined map iteration order.

R=rsc, iant, iant
CC=golang-dev
https://golang.org/cl/5297048

src/pkg/go/ast/print_test.go

@@ -23,11 +23,10 @@ var tests = []struct {
 	{"foobar", "0  \"foobar\""},
 
 	// maps
-	{map[string]int{"a": 1, "b": 2},
+	{map[string]int{"a": 1},
-		`0  map[string] int (len = 2) {
+		`0  map[string] int (len = 1) {
 		1  .  "a": 1
-		2  .  "b": 2
+		2  }`},
-		3  }`},
 
 	// pointers
 	{new(int), "0  *0"},