[aidangarske]

• An unlabeled PR ran the MIMXRT700 M33MU smoke tier, starting 14 emulator jobs on every PR.
• MIMXRT700 now runs on a PR only with the ci:rt700, ci:all, or ci:m33mu label.
• STM32H563 still runs its smoke tier on every PR; ci:h5 runs only the H5 full matrix.
• Pushes to main, the nightly run, and manual dispatch still run both full matrices.
• make test-target TARGET=mimxrt700 still runs the RT700 smoke set locally.
• Updated docs/Testing.md, the workflows README, and the m33mu.yml comments to match.
• Tested with scenario_matrix.py --selftest (ok) and --ci-plan for each label: none gives H5 smoke only (12 jobs), ci:rt700
  gives RT700 full (44) plus H5 smoke, ci:all gives both full matrices.

[Copilot]

🟢 Approval recommended

No active review issues are reported for the change making MIMXRT700 scenarios opt-in on pull requests.

No open issues 🎉

What changed in this PR

This PR makes MIMXRT700 M33MU scenario jobs opt-in on pull requests, reducing default emulator workload while preserving STM32H563 smoke coverage.

Changes:

• Requires ci:rt700, ci:all, or ci:m33mu for MIMXRT700 scenarios on pull requests.
• Updates planner self-test expectations while leaving push, nightly, manual-dispatch, and local smoke behavior unchanged.
• Aligns testing documentation and workflow comments with the new label policy.

File	Description
tests/​target/​lib/​scenario_matrix.py	Adds label-only PR selection for MIMXRT700 and updates self-test expectations.
docs/​Testing.md	Documents revised PR coverage and qualifying labels.
.github/​workflows/​README.md	Updates CI tier descriptions and label effects.
.github/​workflows/​m33mu.yml	Updates comments to reflect the new policy; workflow logic is unchanged.

_{Review effort: Balanced}