test: Improve robustness via use-case tests

[wkz]

Since use-case tests are intended to focus on testing of real-world deployments, it seems prudent to add a reboot phase to ensure that setups work as expected from a cold reboot.

This could hopefully help us catch issues like the recent umask(2) mishap (see #1625).

[troglobit]

CCB: Agreed, possibly also make a config change after reboot to see that the function remains. Target 26.10