Undefined behavior in program spoof tests

[brooksprumo]

Problem

When upgrading nightly Rust¹, a new build error² surfaced:

error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
  --> rust/spoof1/src/lib.rs:27:9
   |
27 |         std::ptr::write_volatile(owner_addr as *mut [u8; 32], fake_system.owner.to_bytes());
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
   = note: `#[deny(invalid_reference_casting)]` on by default
error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
  --> rust/spoof1/src/lib.rs:49:9
   |
49 |         std::ptr::write_volatile(owner_addr as *mut [u8; 32], tmp_native_owner);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
error: could not compile `solana-sbf-rust-spoof1` (lib) due to 2 previous errors

When looking at the original PR that added the spoof tests³, it was unclear to me what the original intent of the tests was, and thus unclear what the obvious fix should be.

Proposed Solution

Fix the undefined behavior.

Footnotes

1. https://github.com/solana-labs/solana/pull/34673 ↩

2. https://buildkite.com/solana-labs/solana/builds/106384#018ce951-65cf-43cf-b89d-ab17ff8a3618 ↩

3. https://github.com/solana-labs/solana/pull/13866 ↩

[brooksprumo]

@alessandrod Is this another one I can send your way?