rust: add kunitconfig

The kunitconfig file in a directory is used by kunit.py to enable all
necessary kernel configurations to run the tests in that subdirectory.
Add such a file for rust/.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20250208-rust-kunit-v1-2-94a026be6d72@weissschuh.net


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