feat: conditionally run unit tests

[erikburt]

Changes

• Introduce composite action setup-ci-core-tests, which consolidates the some of setup for the tests
  • Not all steps can be consolidated because nesting reusable action calls has some weird side effects
• Introduce new workflow ci-core-partial which uses the node action from action: go conditional tests .github#709 to only run test binaries with changes
  • This will be run alongside CI-Core for testing purposes until adequately validated and a transition can be made
  • Fuzz and Race tests are included in this, but only on a schedule for now. Once we replace/consolidate this with CI-Core workflow we will run them on PR as well.

Testing

Workflow running all tests, and updating index: https://github.com/smartcontractkit/chainlink/actions/runs/12187711071/job/33999115005?pr=15347
- This workflow ran all tests because the index had been previously updated with hashes from binaries built with coverage flags. Note: this won't happen during normal execution because coverage being enabled actually causes the workflow to skip updating the hash.

Workflow running no tests: https://github.com/smartcontractkit/chainlink/actions/runs/12188016602/job/34000123940?pr=15347

• The previous run updated the index and then no changes were made, so the hashes of the test binaries shouldn't be changed, and nothing should run.

Test package concurrency of 16 seemed more stable than 32.

• Tested this here: test: conditionally run unit tests #15536
• Couldn't get OCR2 tests to pass when test package concurrency was defaulted to the number of cores.

Rollout

Run this alongside existing validation for at least a week. Only after we've confirmed that the new validation mechanism is working as expected should we replace the existing validation.

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RE-3168