runtime: increase attempt count for map iteration order test.

Some builders broke on this test; I'm guessing that was because
this test didn't try hard enough to find a different iteration order.

Update #6719

R=dave
CC=golang-codereviews
https://golang.org/cl/47300043

src/pkg/runtime/map_test.go

@@ -428,7 +428,7 @@ func TestMapIterOrder(t *testing.T) {
 		}
 		first := ord()
 		ok := false
-		for try := 0; try < 5; try++ {
+		for try := 0; try < 100; try++ {
 			if !reflect.DeepEqual(first, ord()) {
 				ok = true
 				break