Consider allowing `PlanCheck` implementations to trigger `(*testing.T).Skip()`

### Description
Plan Checks can currently return an error in `PlanCheckResponse.Error` to trigger the equivalent of `(*testing.T).Error()` and exit a test if the plan fails the assertion made. We should consider adding functionality to allow plan checks the ability to trigger `(*testing.T).Skip()` with a message.

For example, given a data source attribute value, skip the rest of this test. In the real world, this could be as trivial as a configuration such as:

```terraform
data "aws_partition" "test" {}

# ... other resource configuration ...
```

And wanting to skip the test if `data.aws_partition.test.name != "aws"` for services only available in the AWS Commercial partition.

This could prevent provider developers from needing to implement `TestCase.ErrorCheck` based on API error messaging they may not control.

### Proposal
Add a `Skip` string field to the `CheckPlanResponse` struct. Any plan check that sets this field would immediately invoke `(*testing.T).Skip(skipMsg)` with the string provided. All normal cleanup/defer functions should still be run - i.e. post-test destroy and temp file cleanup.

### References
- Dependent on Plan Check functionality #63