Testing review workflows

The best way to test a PullApprove config is on a real PR. To do this, leave a PR review comment in this format:

/pullapprove-test

```yaml
version: 3
groups:
  ...
```

PullApprove will send the results back to the PR in a comment. Note that this is effectively a "dry run", and won't modify any status checks or send out review requests.

PullApprove config test command