XS::APItest::clone_with_stack(); check SI RC state

XS::APItest::clone_with_stack(); check SI RC state

In this test function, push a couple of stackinfos, one with a
ref-counted stack, and one without.
After cloning, pop the cloned stackinfos and assert that the cloned
argument stacks are also appropriately RC or non-RC.

This should probably have been done properly as some new tests rather
than a couple of asserts, but that would have involved a lot more
effort.