feat: buildkite output format

[laurynaslubys]

This PR introduces two new output formats that properly collapse test output on buildkite. There's a quitet variant (--buildkite) that suppresses output from passed tests and a verbose one (--buildkite-verbose) that prints everything.

I also added some plumbing to enable overriding the summary for a specific format. This is needed to put the summary into a separate section in buildkite rather that under the last test output.

Addresses #316 partly