tests: factor out directory cleanup+creation

tests: factor out directory cleanup+creation

This moves the clean-up to happen up-front. That way one can still
inspect the test data after, e.g., a failed test.

Originally done almost like this in a patch from Fiona [0] that I just
overlooked, but now also factored out, avoid crowding the test code to
much with duplicate code.

[0]: https://lists.proxmox.com/pipermail/pve-devel/2023-June/057136.html

Suggested-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>