Migrate to SciMLTesting v1.2 (off ReTestItems)

[ChrisRackauckas-Claude]

What

Finishes the migration of NonlinearSolve.jl's test harness off ReTestItems/TestItemRunner onto SciMLTesting v1.2.

The root and every sublibrary runtests.jl had already been moved off @testitem/ReTestItems.runtests/@run_package_tests onto a hand-written SafeTestsets group-dispatch ladder (a const GROUP = get(ENV, ...) + if GROUP == ... block + bespoke activate_*_env helpers). This PR replaces those ladders with SciMLTesting's run_tests, which owns the GROUP routing, per-group sub-environment activation, and the Julia <1.11 [sources] develop backport.

Why

ReTestItems parallelizes at the wrong level for SciML (which parallelizes per-GROUP at the CI-matrix level), breaks on new Julia releases, and causes upgrade churn. SciMLTesting centralizes the boilerplate so each runtests.jl is a single declarative run_tests(...) call.

Changes

• test/runtests.jl and lib/*/test/runtests.jl → using SciMLTesting; run_tests(...) in explicit-args mode. The group bodies (file includes / @safetestset blocks) are preserved verbatim, so the same tests run under each GROUP — this is behavior-preserving.
  • Explicit-args (not folder-discovery) is used throughout because the Core folders mix aggregate manifests (core_tests.jl), per-item files (core_tests__itemN.jl), and shared setup_*.jl fixtures that folder-discovery would glob as standalone tests.
  • The root keeps its bespoke sublibrary dispatch (the transitive [sources] develop walk for Julia <1.11) and the Julia-1.12-gated Trim group, which don't fit folder-discovery.
• Test deps: removed Pkg and the now-unused Hwloc (a ReTestItems/nworkers leftover) from each sublibrary test target; added SciMLTesting to every test target. The root keeps Pkg (used directly by the sublibrary/Trim dispatch block).
• test_groups.toml files are unchanged — the group keys already match the group names (Core/NoPre/Verbosity/Downstream/Bounds/Adjoint/Wrappers/CUDA/QA/Trim), and there is no literal All key to rename.

Validation

• Statically: every Project.toml/test_groups.toml parses; every runtests.jl parses with no error/incomplete nodes; all changed .jl files are Runic-clean.
• Runtime (Julia 1.11): SciMLJacobianOperators Core and QA groups both report Testing SciMLJacobianOperators tests passed (Core: ~3600 passing tests; QA: Aqua 11/11 + ExplicitImports 3/3) — exercising the run_tests explicit-args core/qa paths, the sub-env activation, and resolving SciMLTesting from General. Per-PR CI validates the remaining groups across the matrix.

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Ignore until reviewed by @ChrisRackauckas.