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.

1 files changed, 3 insertions, 1 deletions

diff --git a/test/ruby/test_gc.rb b/test/ruby/test_gc.rb
index a1229fc87a..e9b452c702 100644
--- a/test/ruby/test_gc.rb
+++ b/test/ruby/test_gc.rb
@@ -703,7 +703,9 @@ class TestGc < Test::Unit::TestCase
         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