use &raw mut instead of casting &mut to raw pointer

[hkBst]

This gets rid of:

warning: creating a mutable reference to mutable static
  --> src/test_helpers.rs:36:25
   |
36 |     TEST_STATIC_PTR == (&mut TEST_STATIC_PTR as *mut *mut _ as *mut _)
   |                         ^^^^^^^^^^^^^^^^^^^^ mutable reference to mutable static
   |
   = note: mutable references to mutable statics are dangerous; it's undefined behavior if any other pointer to the static is used or if any other reference is created for the static while the mutable reference lives
   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/static-mut-references.html>
   = note: `#[warn(static_mut_refs)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
help: use `&raw mut` instead to create a raw pointer
   |
36 |     TEST_STATIC_PTR == (&raw mut TEST_STATIC_PTR as *mut *mut _ as *mut _)