rust: init: re-enable doctests

Commit a30e94c2 ("rust: init: make doctests compilable/testable")
made these tests buildable among others, but eventually the pin-init
crate was made into its own crate [1] and the tests were marked as
`ignore` in commit 206dea39 ("rust: init: disable doctests").

A few other bits got changed in that reorganization, e.g. the
`clippy::missing_safety_doc` was removed and the `expect` use.

Since there is no reason not to build/test them, re-enable them.

In order to do so, tweak a few bits to keep the build clean, and also use
again `expect` since this is one of those places where we can actually
do so.

Link: https://lore.kernel.org/all/20250308110339.2997091-1-benno.lossin@proton.me/

 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/all/20250526152914.2453949-1-ojeda@kernel.org


Signed-off-by: Benno Lossin <lossin@kernel.org>