Currently there doesn't appear to be a way to get access to all test results in the post step of a pipeline if you had called junit in an earlier stage in the pipeline. TestResultSummary was only availible when junit is called. There may also be multiple TestResultSummary's created when using parallel steps.

This means one can't report the number of tests that have failed/passed if your messaging is done in the final post block of a declarative pipeline.