tests: "We need to get a mut reference to `self.container`." from non-mut &self reference is UB

[decathorpe]

The code that implements the write_bytes method in the impl Bytes<usize> for MockBytesContainer block here relies on undefined behaviour, in particular, creating a &mut reference to data behind a non-mut reference is bad, since it violates the safety contract of non-mut references.

This code happens to have "worked" until current stable Rust / LLVM 15, but starts to exhibit weird failures with future Rust that will use LLVM 16. If this code really needs to rely on "interior mutability", then self.container should probably wrap the backing data in a RefCell or a Mutex to avoid relying on unsafe / undefined behaviour.

[cuviper]

Should be fixed by #228.

[decathorpe]

Was indeed fixed by #228.