Fix test_gc_parameter_init_slots

If the stack is not cleared (e.g. compiling with -O0), then `ary` could
remain on the stack, which would be marked. Clear the array first to
make sure all the objects can be GC'd.

test/ruby/test_gc.rb

@@ -438,6 +438,7 @@ def test_gc_parameter_init_slots
       while GC.stat_heap(0, :heap_allocatable_pages) != 0
         ary << Object.new
       end
+      ary.clear
       ary = nil
 
       # Clear all the objects that were allocated.