Add buildkite-test-collector for testsuite setup

[pv97]

Description

Add buildkite according to https://buildkite.com/organizations/tpu-commons/analytics/suites/tests/setup

In vllm repo these are added to test requirements.txt, there doesn't seem to be equivalent test file in tpu https://github.com/search?q=repo%3Avllm-project%2Fvllm%20buildkite-test-collector&type=code

If the change fixes a bug or a Github issue, please include a link, e.g.,:
b/434301829

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[pv97]

Is there somewhere test requirements are specified?