nested: fix compiling tests with Rust < 1.74

[decathorpe]

The From implementation that this code relies on was only stabilized in Rust 1.74. Changing the .into() call to .to_vec() is equivalent but works on all recent versions of Rust.

error[E0277]: the trait bound `Vec<u8>: From<&[u8; 5]>` is not satisfied
    --> nested/src/lib.rs:1750:38
     |
1750 |             Value::Binary(Cow::Owned(b"owned".into())),
     |                                      ^^^^^^^^ ---- required by a bound introduced by this call
     |                                      |
     |                                      the trait `From<&[u8; 5]>` is not implemented for `Vec<u8>`
     |
     = note: required for `&[u8; 5]` to implement `Into<Vec<u8>>`
help: consider dereferencing here
     |
1750 |             Value::Binary(Cow::Owned(*b"owned".into())),
     |                                      +

c.f. https://blog.rust-lang.org/2023/11/16/Rust-1.74.0.html#stabilized-apis

[KodrAus]

Thanks @decathorpe!