tests/util: add a convenience wrapper to run a ucmd with root permissions

[jhscheer]

As far as I can tell the CICD tests do not run as root, however some functionality can only be tested with root permissions.
While tinkering with #2689 it became apparent to me that there's a need for a a convenience wrapper to run a ucmd with root permissions.
Please let me know if there's a better way to do this.

[sylvestre]

Fails with:

failures:

---- common::util::tests::test_run_ucmd_as_root stdout ----
current_directory_resolved: 
run: sudo -E --non-interactive whoami
thread 'common::util::tests::test_run_ucmd_as_root' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', tests/common/util.rs:965:14

[sylvestre]

For some reasons, I cannot restart the jobs for this tasks?!
could you please rebase it? rivy pushed some changes

[jhscheer]

@sylvestre done

[sylvestre]

Many tests in the code coverage ci are failing

[sylvestre]

``768 failed tests``` it is a bit much :)

[tertsdiepraam]

@sylvestre Any reason this hasn't been merged yet? The CI looks good now.

[sylvestre]

@tertsdiepraam "CICD / Code Coverage (ubuntu-latest, unix) (pull_request) " are failing :)

[jfinkels]

This would help me with #3438

[sylvestre]

sorry for the latency!