Migrate test runner from ReTestItems to standard GROUP-based includes

[ChrisRackauckas-Claude]

Summary

• Replace ReTestItems.jl with a lightweight test/testitem_compat.jl shim that provides @testsetup/@testitem macros, using SafeTestsets-style module isolation
• Rewrite test/runtests.jl to use GROUP-based file inclusion (same pattern as OrdinaryDiffEq.jl)
• Remove ReTestItems and Hwloc from Project.toml

This resolves CI issues like "Package ReTestItems not found" errors while preserving all existing tests unchanged. The @testitem/@testsetup annotations are kept in test files so the VSCode Julia extension can still discover tests.

Design details

@testsetup module Name ... end: Evaluates the module at Main scope. Exports make names available via using.

@testitem "name" [tags=...] [setup=[...]] begin ... end: Creates an isolated module inside a @testset block, automatically injecting using Test, using NeuralPDE, and using ..SetupModule for each setup module. When GROUP != "all", items whose tags don't match the current group are skipped (matching ReTestItems' tag-filtering behavior).

What does NOT change

• No test file modifications — compatibility macros handle everything
• No CI workflow changes — Tests.yml already uses GROUP env var with the same group names
• All tests preserved — no downstream test removal

Test plan

Verified locally on 4 representative test groups:

• GROUP=QA — Aqua + ExplicitImports (catches import issues)
• GROUP=NNPDE2 — direct_function_tests + additional_loss_tests (verifies tag filtering skips untagged items)
• GROUP=AdaptiveLoss — adaptive_loss_tests (verifies @testsetup module mechanism)
• GROUP=DGM — dgm_tests (verifies include("burger_reference_data.jl") resolution in @eval'd modules)
• Runic formatting check on modified files

🤖 Generated with Claude Code