Simplify check for data Rc in account info data and improve tesing

[seanyoung]

sol_invoke_signed_rust() takes an slice of AccountInfo. The account data field is defined as a Rc<RefCell<&mut [u8]>. This means that the AccountInfo struct holds a single pointer for the data field, which points to a struct containing:

• Rc fields for strong and weak reference count
• borrow count field for the RefCell
• a pointer and length for the &[u8] slice

account_info.data.as_ptr() gives as the pointer to the data field of the Rc<RefCell<>> struct, not the data pointer itself; it's the data of the RefCell.

The check for the offset of the length field is therefore redundant and therefore removed.

Also improve the test which show UB without the checks we added in #3471

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.