Show slowest tests

## Summary

Allow to display slowest tests for a given assembly.

## Background and Motivation

On our internal test runs we have an option to display slowest tests. I think this would be a useful feature to provide to users.

## Proposed Feature

Add a `--show-slowest` or `--show-slowest-test` option.

The issue names no files or tests. Start by locating the assembly-scoped command-line option handling and the existing test-result display path; done means the proposed option is accepted and the slowest tests for that assembly are displayed, with coverage for the behavior.

Written by the indexing model from the issue text.