Failing pytest summaries for flaky tests that eventually succeed are not always cleaned up properly: https://buildkite.com/ray-project/ray-builders-branch/builds/7292#_
This PR ensures we only print summaries when we have at least one summary file (and not just the header file).
It is sometimes hard to find all failing tests in buildkite output logs - even filtering for "FAILED" is cumbersome as the output can be overloaded. This PR adds a small utility to add a short summary log in a separate output section at the end of the buildkite job.
The only shared directory between the Buildkite host machine and the test docker container is `/tmp/artifacts:/artifact-mount`. Thus, we write the summary file to this directory, and delete it before actually uploading it as an artifact in the `post-commands` hook.
