🐛 Bug Report — Runtime APIs: `export default`ed `test` handler not found if not last entry

Hey! 👋 With the following `workerd` configuration...

```capnp
// config.capnp
using Workerd = import "/workerd/workerd.capnp";

const unitTests :Workerd.Config = (
services = [
( name = "test",
worker = (
modules = [
( name = "test.mjs", esModule = embed "test.mjs" )
],
bindings = [
( name = "SERVICE", service = "test" )
],
compatibilityDate = "2023-08-01",
)
),
],
);
```

```js
// test.mjs
export default {
async test(ctrl, env, ctx) {
const response = await env.SERVICE.fetch("http://placeholder/");
if (await response.text() !== "body") throw new Error("Aaah!");
},
async fetch(request, env, ctx) {
return new Response("body");
},
}
```

Running `workerd test config.capnp` results in...

```
workerd/server/server.c++:2972: error: No tests found!
Tests failed!
```

However, moving the `fetch` handler before `test` in the `export`ed object results in...

```
workerd/server/server.c++:2941: info: [ TEST ] test
workerd/server/server.c++:2949: info: [ PASS ] test
```

...as expected. Adding an arbitrary entry to the end of the object like `a: 1`, again results in `No tests found!`.