rust: kunit: replace `kernel::c_str!` with C-Strings

rust: kunit: replace `kernel::c_str!` with C-Strings

C-String literals were added in Rust 1.77. Replace instances of
`kernel::c_str!` with C-String literals where possible.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://patch.msgid.link/20251222-cstr-kunit-v1-1-39d999672f35@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>