Documentation: rust: testing: add docs on the new KUnit `#[test]` tests

There was no documentation yet on the KUnit-based `#[test]`s.

Thus add it now.

It includes an explanation about the `assert*!` macros being mapped to
KUnit and the support for `-> Result` introduced in these series.

Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250502215133.1923676-8-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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