rust: add `kunit_tests` to the prelude

It is convenient to have certain things in the `kernel` prelude, and
means kernel developers will find it even easier to start writing tests.

And, anyway, nobody should need to use this identifier for anything else.

Thus add it to the prelude.

Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250502215133.1923676-4-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>