tests/resource/aws_security_group: Ensure sweeper runs after aws_subnet sweeper
tests/resource/aws_security_group: Ensure sweeper runs after aws_subnet sweeper

Otherwise can receive errors like the following, in this case caused by dangling aws_instance resources:

```
2019/01/05 20:10:15 [ERR] error running (aws_security_group): Error deleting Security Group (sg-85dacae1): DependencyViolation: resource sg-85dacae1 has a dependent object
```

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