Deprecated method `tempfile::TempDir::into_path` in tests and examples

[1298345095]

Hi there,

I encountered a deprecation warning regarding tempfile::TempDir::into_path() when running the project.

It seems we should migrate to keep() according to the documentation. I've checked the code and the fix seems straightforward.

I'm happy to submit a PR to fix this. Let me know if that's okay:)

[daviddrysdale]

Happy to review a PR, thanks.

(Looks like the deprecation, and addition of TempDir::keep, arrived in tempfile 3.20.0 so any minimum versions in Cargo.toml file will need updating.)