Use SciMLTesting v1.2 (folder-based run_tests)

[ChrisRackauckas-Claude]

Converts the test suite to the SciMLTesting v1.2 folder-discovery model.

• test/runtests.jl is now just using SciMLTesting; run_tests().
• Core = top-level test/*.jl (core_tests.jl, analytical_tests.jl, interface_tests.jl), extracted self-contained from the canonicalize-safetestset work.
• QA = test/qa/ (its own Project.toml sub-env), running alloc_tests.jl, explicit_imports.jl, jet_tests.jl directly; the old qa.jl includer is removed so folder-discovery runs each file once.
• Root [extras]/[targets].test + [compat]: add SciMLTesting and SafeTestsets, drop Pkg (no test/ code uses it — the harness owns all Pkg ops). test/qa/Project.toml gains SciMLTesting/SafeTestsets/Test.
• test/test_groups.toml unchanged.

Per-GROUP behavior verified to be unchanged by driving the real v1.2.0 dispatcher: Core/All -> the 3 top-level files; QA -> the 3 qa/ files.

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Corrected pass for SciMLTesting v1.1.0: added the explicit curated all = ["Core", "QA"] kwarg so the All/default group's membership is reproduced exactly via the v1.1.0 mechanism rather than relying on the legacy fallback. For this repo the original All branch ran both Core and QA, so both keys are listed (in order). Also made the interface_tests.jl include @__DIR__-relative.

Static-verified: TOML.parsefile(Project.toml) and Meta.parseall on runtests.jl/qa.jl/interface_tests.jl all pass.

Ignore until reviewed by @ChrisRackauckas.