Use SciMLTesting v1.0.0 (run_tests harness)

[ChrisRackauckas-Claude]

Single-package, multi-group (Core/Downstream/QA) shape with a standalone test/Project.toml.

• core thunk runs the 8 Core safetestsets under All/Core.
• Downstream ran under both "All" and "Downstream" in the original, so it is an env-less self-activating thunk (calls activate_group_env(test/downstream) inside the body, env undeclared) — keeping it in-process so it still runs under "All".
• QA ran only for GROUP=="QA", so it declares its test/qa sub-env (excluded from "All"). The custom @test_broken Aqua/JET body in qa/qa.jl is preserved verbatim (not collapsed into run_qa).

Pkg removed from test/Project.toml [deps] (only the old harness used it). The test/qa sub-env is left untouched.

Minor, deliberate, documented refinement: activate_group_env develops the repo into the qa env on Julia 1.10 too, so QA tools analyze the PR-branch source on the LTS (the original relied on [sources], which 1.10 ignores → registered release); on 1.11+ behavior is identical. See the inline comment in runtests.jl.
Behavior-equivalent harness refactor: replace the hand-written GROUP-dispatch in test/runtests.jl with a single SciMLTesting.run_tests call (SciMLTesting v1.0.0), and add SciMLTesting to the test dependencies. No test assertions or logic were changed.

Verified locally:

• TOML.parsefile on the edited Project.toml(s) confirms SciMLTesting is in the test deps with the correct UUID, and Pkg removed where it was only used by the old harness.
• Meta.parseall on the new runtests.jl is clean (no error nodes).
• SciMLTesting v1.0.0 resolves from the registry alongside the existing test deps.
• The run_tests group/qa argument shapes validate against SciMLTesting's internal GroupSpec normalizer.

This PR is a draft and should be ignored until reviewed by @ChrisRackauckas.

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