Use SciMLTesting v1.2 (folder-based run_tests)

[ChrisRackauckas-Claude]

Converts test/runtests.jl to the SciMLTesting v1.2 folder-discovery model: the whole file becomes

using SciMLTesting
run_tests()

run_tests() reads test/test_groups.toml (unchanged) and discovers each group's files from the folder layout, running every file in its own isolated @safetestset. This is behavior-equivalent to the old hand-written GROUP dispatch.

Folder mapping (behavior-preserving)

• Core (GROUP == "Core", and the Core portion of "All"): the top-level test/*.jl files — DataReduction.jl, deim.jl, utils.jl — exactly the three files the old GROUP == "All" || "Core" block ran (as POD/utils/DEIM). They stay at the top level (no move) and run in the main test env.
• QA (GROUP == "QA"): the existing test/qa/ folder — qa_tests.jl, explicit_imports_tests.jl, jet_tests.jl — the same three files the old GROUP == "All" || "QA" block ran. The folder carries its own test/qa/Project.toml, so the harness activate_group_env(test/qa) does Pkg.activate + Pkg.develop(repo root) + Pkg.instantiate(), equivalent to the old manual Pkg.activate/Pkg.develop/Pkg.instantiate.

test/test_groups.toml is unchanged ([Core], [QA]); the CI matrix runs Core and QA as separate lanes, matching the harness mapping.

Project.toml

• test/Project.toml: add SciMLTesting = "09d9d899-5365-40a9-917a-5f67fddea283" (+ [compat] SciMLTesting = "1"); drop Pkg (deps + compat) — only the old harness used it (no test/*.jl references Pkg.; the v1.2 harness owns all Pkg operations). SafeTestsets and Test retained.
• test/qa/Project.toml: add SciMLTesting (+compat "1"), SafeTestsets (+compat "0.0.1, 0.1"), Test (+compat "1") so the folder-discovered files run under @safetestset in the QA sub-env. Added a relative [sources] ModelOrderReduction = {path = "../.."} (belt-and-suspenders for Julia 1.11+; activate_group_env also develops the package).

Verification

• TOML.parsefile on every Project.toml + test_groups.toml; asserted SciMLTesting UUID/compat present, Pkg removed from test deps, test_groups.toml keys unchanged.
• Meta.parseall on runtests.jl and every test file: no parse errors.
• Drove SciMLTesting v1.2.0's folder-discovery resolution: Core → {DataReduction, deim, utils}, QA → {qa_tests, explicit_imports_tests, jet_tests} with sub-env activation, All → Core (QA excluded by design, run as its own CI lane). Matches the old dispatcher.

Heavy suite not run locally; per-PR CI validates.

Ignore until reviewed by @ChrisRackauckas.