Minor: Update output validity tests (#13190)

* 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

substrate/client/executor/src/integration_tests/mod.rs

@@ -811,7 +811,7 @@ fn return_max_memory_offset(wasm_method: WasmExecutionMethod) {
 
 	assert_eq!(
 		call_in_wasm("test_return_max_memory_offset", &[], wasm_method, &mut ext).unwrap(),
-		().encode()
+		(u8::MAX).encode()
 	);
 }