Use SciMLTesting v1.2 (folder-based run_tests)

[ChrisRackauckas-Claude]

Convert the test suite to the SciMLTesting v1.2 folder-discovery model. test/runtests.jl becomes:

using SciMLTesting
run_tests()

Layout

• Core = top-level test/*.jl (hamiltonian_test.jl, explicit_imports.jl), both already self-contained.
• test/shared/ (a non-group folder, never auto-discovered) holds the commented-out nbody_test.jl, keeping the discovered Core set equal to the prior dispatch.
• QA = test/qa/ with its existing sub-env Project.toml.

Deps

• Add SciMLTesting to the root test target; add SciMLTesting + SafeTestsets to the QA sub-env Project.toml.
• Bump SafeTestsets compat to "0.1, 1".
• Drop Pkg from the root test deps (only the old harness used it).
• test/test_groups.toml unchanged.

Verification

• Static: every Project.toml + test_groups.toml parses; every test .jl parses; folder-discovery (real SciMLTesting v1.2.0) resolves Core/All to explicit_imports.jl + hamiltonian_test.jl and QA to qa.jl in its sub-env.
• End-to-end: GROUP=QA Pkg.test() on Julia 1.11 passed (6 pass / 3 broken — the 3 pre-existing @test_broken), exercising the sub-env activation path.
• Runic-formatted.

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Force-updated this branch to the corrected SciMLTesting v1.1.0 conversion, superseding the v1.0.0 first pass.

What changed vs the v1.0.0 commit on this branch:

• QA fixed. v1.0.0 put "QA" in groups=, but "QA" is a reserved key run_tests intercepts before the groups table — with no qa= body, GROUP=QA threw ArgumentError. Now QA uses the reserved qa= kwarg and is selectable by name.
• Core body is now a file path (test/core_tests.jl) instead of an inline () -> begin ... end thunk (the v1.1.0 world-age-safe, isolated @safetestset form).
• Explicit all = ["Core"] documents and pins the All membership (Core only, QA omitted) rather than relying on the legacy fallback.

Behavior is identical to upstream master for every GROUP value (bare/All, Core, QA). Static-verified: TOML.parsefile on both Project.toml files and Meta.parseall on runtests.jl/core_tests.jl/qa/qa.jl all clean.