rust: enable macros::module! tests

rust: enable macros::module! tests

Previously, these tests were ignored due to a missing necessary dependency
on the `kernel` crate. Enable the tests, and update them: for both,
add the parameter to `init()`; for the first one, remove the use of a
kernel parameter mechanism that was never merged.

Signed-off-by: Ethan D. Twardy <ethan.twardy@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1076
Link: https://lore.kernel.org/r/20240704145607.17732-3-ethan.twardy@gmail.com
[ Rebased (moved the `export` to the `rustdoc_test` rule, enable the
  firmware example too). Removed `export` for `RUST_MODFILE`. Removed
  unneeded `rust` language in examples, as well as `#[macro_use]`
  `extern`s. Reworded accordingly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>