* Minor: Update output validity tests

Quick follow-up to https://github.com/paritytech/substrate/pull/13183.

Mainly, I wanted to double check that the `test_return_max_memory_offset` test doesn't pass just
because the output length is 0.

I also:

- Organized these tests into a module.
- Added a comment explaining why we don't use the `wasm_export_functions` macro.

* Update test based on review comment