Fix flakiness in TestGc#test_thrashing_for_young_objects

I caught a reproduction of this test failing under rr, and was able to
replay it to isolate the failure. The call to
`before_stat_heap = GC.stat_heap` is itself allocating a hash, which in
unlucky circumstances can result in a new page being allocated and thus
`before_stats[:heap_allocated_pages]` no longer equals
`after_stats[:heap_allocated_pages]`.

The solution is to use the form of GC.stat/stat_heap which takes a hash
as an argument, and thus needs to perform no Ruby allocations itself.

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