tests/resource/aws_subnet: Add aws_msk_cluster to test sweeper dependencies
tests/resource/aws_subnet: Add aws_msk_cluster to test sweeper dependencies

To help prevent:

```
[19:01:16][Step 2/5] 2019/07/02 19:01:16 [ERR] error running (aws_internet_gateway): Error deleting Subnet (subnet-04ce94125fcb69849): DependencyViolation: The subnet 'subnet-04ce94125fcb69849' has dependencies and cannot be deleted.
[19:01:16][Step 2/5] 	status code: 400, request id: 547657de-8b91-4088-8dd0-24889d868449
```

kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws