Reuse and overwritting of node logs during tests

[cjen1-msft]

https://github.com/microsoft/CCF/actions/runs/18711838663/job/53362454045?pr=7189

We should be more careful to ensure that we don't overwrite a previously existing test directory.
On sane disks this will make debugging after the fact potentially more difficult, but if the deletion is non-atomic for any reason, then when we recreate the folder we may end up with pre-existing files still existing...