test: Add tests for issue #12223, "drop allowed while active borrows
still in scope".

This issue was fixed by PR #12828 and #5781. All that was left was to
add tests.

Closes #12223.
auto merge of #15310 : pcwalton/rust/tests-for-12223, r=brson

still in scope".

This issue was fixed by PR #12828 and #5781. All that was left was to
add tests.

Closes #12223.