tests/resource/aws_db_snapshot: Ensure sweeper runs after aws_db_instance and skips automated snapshots (#13897)
tests/resource/aws_db_snapshot: Ensure sweeper runs after aws_db_instance and skips automated snapshots (#13897)

Reference: https://github.com/terraform-providers/terraform-provider-aws/issues/13800

Looks like the `aws_db_instance` sweeper separately needs to be updated to handle API deletion protection.

Output from sweeper in AWS Commercial:

```
2020/06/23 09:05:56 [DEBUG] Running Sweepers for region (us-west-2):
2020/06/23 09:05:56 [DEBUG] Running Sweeper (aws_db_instance) in region (us-west-2)
...
2020/06/23 09:05:59 [INFO] Deleting DB instance: tf-acc-test-5943499196810915120
2020/06/23 09:06:00 [ERROR] Failed to delete DB instance tf-acc-test-5943499196810915120: InvalidParameterCombination: Cannot delete protected DB Instance, please disable deletion protection and try again.
	status code: 400, request id: 90caa3c3-9f82-4ad5-ad74-a68238240564
2020/06/23 09:06:00 [DEBUG] Running Sweeper (aws_db_snapshot) in region (us-west-2)
2020/06/23 09:06:00 Sweeper Tests ran successfully:
	- aws_db_instance
	- aws_db_snapshot
2020/06/23 09:06:00 [DEBUG] Running Sweepers for region (us-east-1):
2020/06/23 09:06:00 [DEBUG] Running Sweeper (aws_db_instance) in region (us-east-1)
...
2020/06/23 09:06:02 [DEBUG] Running Sweeper (aws_db_snapshot) in region (us-east-1)
2020/06/23 09:06:02 Sweeper Tests ran successfully:
	- aws_db_instance
	- aws_db_snapshot
ok  	github.com/terraform-providers/terraform-provider-aws/aws	6.798s
```

Output from sweeper in AWS GovCloud (US):

```
2020/06/23 09:06:15 [DEBUG] Running Sweepers for region (us-gov-west-1):
2020/06/23 09:06:15 [DEBUG] Running Sweeper (aws_db_instance) in region (us-gov-west-1)
...
2020/06/23 09:06:17 [DEBUG] Running Sweeper (aws_db_snapshot) in region (us-gov-west-1)
2020/06/23 09:06:18 Sweeper Tests ran successfully:
	- aws_db_instance
	- aws_db_snapshot
ok  	github.com/terraform-providers/terraform-provider-aws/aws	4.148s
```
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws