Use tempdir for tests (#993)

* Use tempdir for tests

* Rename tmp to tempdir

* Update Cargo.lock

* Update expect error message in run_command_and_kill

* Call with .path() rather than creating an string slice

* Call tempdir with arg instead of args

* Update tests/purge_chain_works.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>