Assuming EXIT_SUCCESS equals 0 is not portable

Author: nobu

test/ruby/test_fiber.rb

@@ -396,7 +396,7 @@ def test_fork_from_fiber
                 Fiber.new {}.transfer
                 Fiber.new { Fiber.yield }
               end
-              exit!(0)
+              exit!(true)
             end
           }.transfer
           _, status = Process.waitpid2(xpid)
@@ -405,8 +405,8 @@ def test_fork_from_fiber
       end.resume
     end
     pid, status = Process.waitpid2(pid)
-    assert_equal(0, status.exitstatus, bug5700)
-    assert_equal(false, status.signaled?, bug5700)
+    assert_not_predicate(status, :signaled?, bug5700)
+    assert_predicate(status, :success?, bug5700)
   end
 
   def test_exit_in_fiber