rust: kunit: allow `cfg` on `test`s

rust: kunit: allow `cfg` on `test`s

The `kunit_test` proc macro only checks for the `test` attribute
immediately preceding a `fn`. If the function is disabled via a `cfg`,
the generated code would result in a compile error referencing a
non-existent function [1].

This collects attributes and specifically cherry-picks `cfg` attributes
to be duplicated inside KUnit wrapper functions such that a test function
disabled via `cfg` compiles and is marked as skipped in KUnit correctly.

Link: https://lore.kernel.org/r/20250916021259.115578-1-ent3rm4n@gmail.com
Link: https://lore.kernel.org/rust-for-linux/CANiq72==48=69hYiDo1321pCzgn_n1_jg=ez5UYXX91c+g5JVQ@mail.gmail.com/
Closes: https://github.com/Rust-for-Linux/linux/issues/1185
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Suggested-by: David Gow <davidgow@google.com>
Signed-off-by: Kaibo Ma <ent3rm4n@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>