Allow custom policy output to be parsed similarly to that of standard policy check

[Mezage]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story
As a developer, I often want to enable custom policies and use Conftest's ouput, but the logic fails on the standard output conftest produces

Describe the solution you'd like
I'd love for custom policies to be given the option to be parsed based on the standard output that Contest produces (ref pic above)
Since custom policies trigger on the term 'fail', this string will always fail in the eyes of atlantis. I'd love for there to be a flag for custom policy logic to also look for this regex in policy output.

Describe alternatives you've considered
So far my team had to create a function that scrubs "failures" from Conftest's output string if nothing is failing, to avoid false positives. But that results in a blank summary block in the policy GHE comment.