SkipPostTestDestroy option in TestCase

### SDK version
```
github.com/hashicorp/terraform-plugin-sdk/v2 v2.38.1
```

### Use-cases
```
I want to run testcases on resources that are already created.
An example is import a resource that always exists and check whether the existing plan has a diff compared to the imported resource.
```

### Attempted Solutions
```
I have tried to run tests but there is no option to prevent a destroy of the resource after all steps are completed
```

### Proposal
```
Add an option SkipPostTestDestroy that prevents destory of resources in the state after all steps are complete
By default this option is false to ensure backward compatibility
To be added here https://github.com/hashicorp/terraform-plugin-sdk/blob/v2.38.1/helper/resource/testing_new.go#L78

I assume SkipPostTestDestroy is not present to avoid keeping lingering resources
In certain cases, we will need this option
```

### References
```
https://github.com/hashicorp/terraform-plugin-sdk/blob/v2.38.1/helper/resource/testing.go#L308
https://github.com/hashicorp/terraform-plugin-sdk/blob/v2.38.1/helper/resource/testing_new.go#L78
```