rust: kunit: use C-string literals to clean warning

rust: kunit: use C-string literals to clean warning

Starting with upstream Rust commit a5e3a3f9b6bd ("move
`manual_c_str_literals` to complexity"), to be released in Rust 1.83.0
[1], Clippy now warns on `manual_c_str_literals` by default, e.g.:

    error: manually constructing a nul-terminated string
      --> rust/kernel/kunit.rs:21:13
       |
    21 |             b"\x013%pA\0".as_ptr() as _,
       |             ^^^^^^^^^^^^^ help: use a `c""` literal: `c"\x013%pA"`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_c_str_literals
       = note: `-D clippy::manual-c-str-literals` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(clippy::manual_c_str_literals)]`

Apply the suggestion to clean up the warnings.

Link: https://github.com/rust-lang/rust-clippy/pull/13263
Reviewed-by: Trevor Gross <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>