Cancel superseded PR runs in the test workflows

[albertvillanova]

What does this PR do?

build_pr_documentation.yml cancels a PR's in-flight run when a new commit is pushed to the same branch. tests.yml and tests-experimental.yml do not, so every push to an open PR leaves the previous run occupying runners until it finishes on its own, testing a commit nobody will merge.

Over 2026-08-26 to 2026-09-10, 341 PR runs of tests.yml were superseded by a newer commit on the same branch and kept running anyway. 64 GPU-hours of that was spent after the superseding run had already been created. That capacity is shared, so those runs also sit ahead of other PRs in the queue.

tests.yml books five GPU jobs per run (four on aws-g6e-4xlarge, one on aws-g5-12xlarge-cache), around 77 runner-minutes per full matrix.

This adds the same concurrency block the doc workflow already uses, keyed on the pull request number. github.event.pull_request.number is empty for push events, so the group falls back to the unique github.run_id and pushes to main and to ci-* branches are never cancelled. Only PR runs superseded by a newer commit on the same PR are.

The doc workflow keys on github.head_ref, which is only the branch name and is shared across forks, so two PRs that both use a branch called patch-1 would cancel each other. Bugbot caught that on the first version of this PR. No two open PRs share a head branch name today, but 130 of the 182 open ones come from forks, so the collision is reachable and the PR number costs nothing. build_pr_documentation.yml still carries the head_ref form and could be aligned separately.

The 64 hours is the reclaimable part, not the full 352 hours those runs consumed, and it counts tests.yml only, so the estimate is conservative.

Part of #7169.

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Note

Low Risk
CI-only workflow configuration; no application or test logic changes, only earlier termination of obsolete PR runs.

Overview
Adds GitHub Actions concurrency to tests.yml and tests-experimental.yml so an older workflow run is cancelled when a newer commit triggers another run for the same PR.

Each workflow uses group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }} with cancel-in-progress: true, matching the pattern already used for PR doc builds. PRs share one group per workflow via the PR number; non–pull-request triggers fall back to run_id, so pushes to main and ci-* are not cancelled by each other.

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[qgallouedec]

thanks, long due