stack has 1 items remaining

I'm working on a new MoonBit implementation, and I wanted to add runner test for simple-call-import. However, I got this error :

```
stack has 1 items remaining: ["result"]
```

The emitted instructions are the following, which does not seem balanced to me.

```
CallWasm { name: "f", sig: WasmSignature { params: [], results: [I32], indirect_params: false, retptr: false } }
Return { amt: 0, func: Function { name: "f", kind: AsyncFreestanding, params: [], result: None, docs: Docs { contents: None }, stability: Unknown } }
```

To reproduce, with this commit: https://github.com/peter-jerry-ye/wit-bindgen/tree/0b4b5c35bd88b8caf080d69ce6ebd8c03dfcb861
```bash
cargo run test --languages moonbit,rust --artifacts target/artifacts ./tests/runtime-async/async/simple-call-import
```