Fix nondeterministic failure in test_latest_gc_info_weak_references_count

Clear the ary variable before setting it to nil.  Otherwise, if
the previous ary value was somewhere on the stack, all references
in it would be considered live, and the wmap size would be 10000.

Author: jeremyevans

test/ruby/test_gc.rb

@@ -411,6 +411,8 @@ def test_latest_gc_info_weak_references_count
       before_weak_references_count = GC.latest_gc_info(:weak_references_count)
       before_retained_weak_references_count = GC.latest_gc_info(:retained_weak_references_count)
 
+      # Clear ary, so if ary itself is somewhere on the stack, it won't hold all references
+      ary.clear
       ary = nil
 
       # Free ary, which should empty out the wmap