chore(kinesisfirehose-alpha): refactor integ tests with assertions (a…

…ws#32560)

### Description of changes

Add assertions to firehose and firehose-destinations integ tests. 

No assertions were added to `integ.delivery-stream.ts` because the same case is being covered by the `integ.s3-bucket.lit.ts` and `integ.delviery-stream.source-stream.ts` tests. 

### Description of how you validated changes

integ tests

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*