Add a description field in a TestStep.

### terraform-plugin-testing version

```
v1.51.1
```

### Use cases

When I have a complicated test to run, I will have different steps and I would like to print when the test run what will change.
Having a TestStep description field could help to make it more obvious in the test runner what step are running and what change are happening.

### Attempted solutions

### Proposal

Add a description field and make it show up when the test are running

### References