MockPayloadGenerator: __typename mocking depends on generated request ordering

We have a regression between v12 and v13 in example queries using `MockPayloadGenerator`.

This is caused by a bug in MockPayloadGenerator, but appears as a regression when upgrading.

It has to do with the feature where the `__typename` is correctly provided in the mock payload if you don't provide a `__typename` value in the mock data.

Assume we have these 2 selections _generated by the compiler(s)_:
```js
const generalSelection = {
kind: 'InlineFragment',
selections: [
{
kind: 'ScalarField',
name: 'id',
},
],
type: 'TypeB',
};

const typenameSelection = {
kind: 'ScalarField',
name: '__typename',
};
```

For a query that looks like this
```graphql
query testQuery {
a {
b {
... on TypeB {
id
}
__typename
}
}
}
```

These 2 selections are going to be in a request under a linked field

```js
const dummyOperationDescriptor = (items) => ({
request: {
node: { // <-- the generated ConcreteRequest
operation: {
name: 'testQuery',
selections: [
{
concreteType: 'TypeA',
kind: 'LinkedField',
name: 'a',
selections: [
{
concreteType: null,
kind: 'LinkedField',
name: 'b',
plural: false,
selections: items, // <-- here's the order-dependent list
},
],
},
],
},
params: {
metadata: {},
},
},
variables: {},
cacheConfig: {},
},
});
```

The JS compiler always output the `__typename` field first in this list of selections
```js
const working = RelayMockPayloadGenerator.generate(dummyOperationDescriptor([
typenameSelection,
generalSelection
]));
```
Which leads to this (correct) output
```json
{
"data": {
"a": {
"b": {
"__typename": "TypeB",
"id": ""
}
}
}
}
```

The rust compiler changed the output ordering of fields, and `__typename` is no longer first.

```js
const failing = RelayMockPayloadGenerator.generate(dummyOperationDescriptor([
generalSelection,
typenameSelection
]));
```

Now this gives us incorrect output
```json
{
"data": {
"a": {
"b": {
"__typename": "__MockObject",
"id": ""
}
}
}
}
```
This means that queries that we're previously mocked fine no longer work