Persistent failure report

## Overview

It would be good to have a report that highlights persistent failures - checks that consistently fail for many days.

## Requirements

- configurable threshold for persistent failure, measured in time
- report is a list of check, time period of failure pairs & last successful run (`check foo has been failing for 8 days, last successful run $TIMESTAMP`)
- if we were clever, could extend to flapping tests...

## Approach

Write a report in https://github.com/ComplianceAsCode/auditree-arboretum/tree/main/arboretum/auditree/reports

## Security and Privacy

N/A

## Test Plan

Will need some fake result data to demonstrate persistent failure.