Change test_warmup_frees_pages to check each size pool

This should help in debugging the intermittent test failures on CI:

    TestProcess#test_warmup_frees_pages [test/ruby/test_process.rb:2779]:
    <201> expected but was
    <202>.

test/ruby/test_process.rb

@@ -2771,12 +2771,14 @@ def test_warmup_frees_pages
       # Disable GC so we can make sure GC only runs in Process.warmup
       GC.disable
 
-      total_pages_before = GC.stat(:heap_eden_pages) + GC.stat(:heap_allocatable_pages)
+      total_pages_before = GC.stat_heap.map { |_, v| v[:heap_eden_pages] + v[:heap_allocatable_pages] }
 
       Process.warmup
 
       # 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))
+      total_pages_before.each_with_index do |val, i|
+        assert_equal(val, GC.stat_heap(i, :heap_eden_pages) + GC.stat_heap(i, :heap_allocatable_pages), "size pool: #{i}")
+      end
       assert_equal(0, GC.stat(:heap_tomb_pages))
       assert_operator(GC.stat(:total_freed_pages), :>, 0)
     end;