Revise testing of Obj.reachable_words

Part of the test makes sense only if the runtime system supports
naked pointers and has a page table to distinguish major heap pointers
from out-of-heap pointers.

This part is split off in a new test, lib-obj/reachable_words_np.ml,
conditionalized on "naked_pointers".

testsuite/tests/lib-obj/reachable_words.ml

@@ -1,11 +1,6 @@
 (* TEST
 *)
 
-let native =
-  match Sys.backend_type with
-  | Sys.Native -> true
-  | Sys.Bytecode -> false
-  | Sys.Other s -> print_endline s; assert false
 
 
 let size x = Obj.reachable_words (Obj.repr x)
@@ -22,7 +17,6 @@ type t =
 let f () =
   let x = Random.int 10 in
   expect_size 0 42;
-  expect_size (if native then 0 else 3) (1, 2);
   expect_size 2 [| x |];
   expect_size 3 [| x; 0 |];
 

testsuite/tests/lib-obj/reachable_words_np.ml

@@ -0,0 +1,21 @@
+(* TEST
+ * naked_pointers
+ ** bytecode
+ ** native
+*)
+
+let native =
+  match Sys.backend_type with
+  | Sys.Native -> true
+  | Sys.Bytecode -> false
+  | Sys.Other s -> print_endline s; assert false
+
+let size x = Obj.reachable_words (Obj.repr x)
+
+let expect_size s x =
+  let i = size x in
+  if i <> s then
+    Printf.printf "size = %i; expected = %i\n%!" i s
+
+let () =
+  expect_size (if native then 0 else 3) (1, 2)