rust: str: convert `rusttest` tests into KUnit

In general, we should aim to test as much as possible within the actual
kernel, and not in the build host.

Thus convert these `rusttest` tests into KUnit tests.

Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250502215133.1923676-5-ojeda@kernel.org


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