Reduce sizes of tuples of tuples2.scala

The previous version worked with a -Xss2m setting, but it seems stack size
used in tests is significantly smaller than that.

Is there a way to set the stack size for a specific test? Then we could
re-enable the commented-out code.

Author: odersky

tests/pos-deep-subtype/tuples2.scala

@@ -15,6 +15,8 @@ object Test extends App {
   assert(xs3(127) == 16)
     // 3.416s
 
+/* The following operations exhaust the standard stack, but succeed with -Xs10m:
+
   val xs4 = xs3 ++ xs3
   assert(xs4(255) == 16)
     // 3.765s
@@ -23,8 +25,6 @@ object Test extends App {
   assert(xs5a(383) == 16)
     // 3.804s
 
-/* The following operations exhaust the standard 2MB stack, but succeed with -Xs10m:
-
   val xs5 = xs4 ++ xs4
   assert(xs5(511) == 16)
     // 3.866s