Migrate to SciMLTesting v1.2 (off ReTestItems)

[ChrisRackauckas-Claude]

Migrates the test suite off ReTestItems to the SciMLTesting v1.2 folder-discovery model.

Why

ReTestItems parallelizes at the wrong level for SciML (we parallelize at the CI matrix level per GROUP), breaks on new Julia releases, and causes upgrade friction. SciMLTesting's folder-discovery replaces the @testitem + ReTestItems.runtests(...; tags, nworkers) machinery with a declarative run_tests() that maps each GROUP to a folder of plain @testset files, each run in its own @safetestset.

What changed

• test/runtests.jl is now just using SciMLTesting; run_tests().
• @testitem blocks converted to plain self-contained @testset files and placed by group:
  • Core (top-level test/*.jl): native line search custom-optimizer + Newton-Raphson + Strong Wolfe edge cases.
  • LineSearchesJL (test/linesearchesjl/): LineSearches.jl extension custom-optimizer + Newton-Raphson.
  • QA (test/qa/): ExplicitImports, with its own sub-env Project.toml.
• The shared @testsetup modules (CustomOptimizer / RootFinding) are inlined into each file that uses them so every file is self-contained.
• test/test_groups.toml keeps the same CI matrix cells (Core, LineSearchesJL, QA); QA gets in_all = false for clarity.
• Root Project.toml: drops ReTestItems, Hwloc, InteractiveUtils, and the unused NonlinearProblemLibrary; moves ExplicitImports into the qa sub-env; adds SciMLTesting + SafeTestsets to [extras]/[compat]/[targets].test.
• The separate Enzyme CI job (test/enzyme/, run directly via its own Project.toml outside the GROUP mechanism) is unchanged.

This is behavior-preserving: the same tests run under each GROUP after migration; no assertions were altered.

Verification (local, Julia 1.11)

• GROUP=Core: Core/custom_optimizer_tests.jl 144/144, Core/root_finding_tests.jl 85/85 — passed.
• GROUP=LineSearchesJL: custom_optimizer_tests.jl 96/96, root_finding_tests.jl 80/80 — passed.
• GROUP=QA: explicit_imports_test.jl 2/2 — passed.

Ignore until reviewed by @ChrisRackauckas.

🤖 Generated with Claude Code