Restore `dbg!` regression tests

These are all tests from trying to fix the tear-free `dbg!`:

* rust-lang/rust#154074
* rust-lang/rust#154994
* rust-lang/rust#155915

These should also work as we revert to the old `dbg!`, so we should keep
the tests to make sure they don't regress again.

Co-authored-by: dianne <diannes.gm@gmail.com>