Set info about the test results in the env so that post steps can access it

### What feature do you want to see added?

I would like to be able to access the number of failed tests in a post stage where I send a Slack message about the failure of the job. Is there already a way to do this? I am definetly up for doing this myself, is there something I should think about before I start?

### Upstream changes

_No response_

The issue names no files, tests, or entry points. Begin by determining whether the plugin already exposes test-result counts to Jenkins post steps; done means a post stage can access the number of failed tests for a Slack notification, with the expected behavior clarified.

Written by the indexing model from the issue text.