Add debug info for flaky test_warmup_frees_pages

This test sometimes fails with:

```
  1) Failure:
TestProcess#test_warmup_frees_pages [test/ruby/test_process.rb:2751]:
<0> expected but was
<1>.
```

I'm not sure why, so add some debug info.

test/ruby/test_process.rb

@@ -2748,7 +2748,7 @@ def test_warmup_frees_pages
 
       # Number of pages freed should cause equal increase in number of allocatable pages.
       assert_equal(total_pages_before, GC.stat(:heap_eden_pages) + GC.stat(:heap_allocatable_pages))
-      assert_equal(0, GC.stat(:heap_tomb_pages))
+      assert_equal(0, GC.stat(:heap_tomb_pages), GC.stat)
       assert_operator(GC.stat(:total_freed_pages), :>, 0)
     end;
   end