Add debug message to test_heaps_grow_independently

To debug flaky failures on i686 that look like:

  1) Failure:
  TestGc#test_heaps_grow_independently [test/ruby/test_gc.rb:704]:
  Expected 2061929 to be < 2000000.

Author: peterzhu2118

test/ruby/test_gc.rb

@@ -703,7 +703,9 @@ def allocate_large_object = Array.new(10)
         allocate_large_object
       end
 
-      assert_operator(GC.stat(:heap_available_slots), :<, COUNT * 2)
+      heap_available_slots = GC.stat(:heap_available_slots)
+
+      assert_operator(heap_available_slots, :<, COUNT * 2, "GC.stat: #{GC.stat}\nGC.stat_heap: #{GC.stat_heap}")
     RUBY
   end