Unable to write unit tests for StackSetStacks

I've been trying to write a CDK unit test for my `StackSetStack` but I keep running into issues with this no matter how I structure it.

**Targeting the Parent Stack**

With this setup, the test runs but the template that is synthesized is not of any interest to me as it only contains the inner workings (Lambda, CustomResource, Assets, etc.) that makes `cdk-stacksets` function but it doesn't contain any of the details of my `MyStackSetStack` that I would like to test.

```typescript
it('matches the snapshot', () => {
const app = new App();
const stack = new Stack(app, 'TestStack');
const stackSetStack = new MyStackSetStack(stack, 'MyStackSetStack', {
assetBuckets: [Bucket.fromBucketName(stack, 'Assets', 'stackset-assets-us-east-2')],
assetBucketPrefix: 'stackset-assets',
});

const template = Template.fromStack(stack);

expect(template).toMatchSnapshot();
});
```

**Targeting the StackSetStack**

Since `StackSetStack` extends `Stack`, I figured I could just switch `Template.fromStack` to target the `stackSetStack` directly and then I would get the template I want to run my tests against.

But with this setup, I always get this error:

`Unable to find artifact with id "TestStackMyStackSetStackXXXXXXXX"`

```typescript
const app = new App();
const stack = new Stack(app, 'TestStack');
const stackSetStack = new MyStackSetStack(stack, 'MyStackSetStack', {
assetBuckets: [Bucket.fromBucketName(stack, 'Assets', 'stackset-assets-us-east-2')],
assetBucketPrefix: 'stackset-assets',
});

const template = Template.fromStack(stackSetStack);

expect(template).toMatchSnapshot();
```

**Removing Parent Stack All Together**

Next I tried to just remove the Parent stack entirely and just create the `MyStackSetStack` on the app itself.

But this gave me this error because of the asset bucket:

`App at '' should be created in the scope of a Stack, but no Stack found`


```typescript
const app = new App();
const stackSetStack = new MyStackSetStack(app, 'MyStackSetStack', {
assetBuckets: [Bucket.fromBucketName(app, 'Assets', 'stackset-assets-us-east-2')],
assetBucketPrefix: 'stackset-assets',
});

const template = Template.fromStack(stackSetStack);

expect(template).toMatchSnapshot();
```

**Removing Asset Bucket**

So my last attempt was to do it without the asset bucket but this resulted in this error:


```typescript
const app = new App();
const stackSetStack = new MyStackSetStack(app, 'MyStackSetStack', {
assetBuckets: [Bucket.fromBucketName(app, 'Assets', 'stackset-assets-us-east-2')],
assetBucketPrefix: 'stackset-assets',
});

const template = Template.fromStack(stackSetStack);

expect(template).toMatchSnapshot();
```

---

Again, since the `StackSetStack` is a `Stack`, it would be nice if we could test them just like we do any other AWS CDK stack.