Use SciMLTesting v1.1.0 run_tests harness

[ChrisRackauckas-Claude]

Replaces the hand-written GROUP if-ladder in test/runtests.jl with a single declarative run_tests call from SciMLTesting v1.1.0.

What changed

• The six functional groups (Modeling, Simulation, Hybrid, Misc, Spatial, Extensions) are passed as groups entries.
• A curated all = ["Modeling", "Simulation", "Hybrid", "Misc", "Spatial", "Extensions"] reproduces the original All membership exactly: bare/All runs all six groups in the original order. This repo has no QA group.
• The Extensions group keeps its isolated test/extensions environment via the env kwarg, which performs the same Pkg.activate / Pkg.develop(repo root) / Pkg.instantiate that the old activate_extensions_env() helper did.

Behavior equivalence

This is a behavior-equivalent refactor — no test assertions or logic changed, and the set of testsets run under each GROUP value (including bare/All) is identical before and after. Verified with a dispatch dry-run:

GROUP	groups run
All	Modeling, Simulation, Hybrid, Misc, Spatial, Extensions
Modeling / Simulation / Hybrid / Misc / Spatial / Extensions	the named group only

Project.toml

• Added SciMLTesting (09d9d899-5365-40a9-917a-5f67fddea283) to [extras], [compat] SciMLTesting = "1", and the test target.
• Dropped Pkg from the test deps — only the old harness used it; run_tests bundles Pkg.

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Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Rebased onto current master (picks up #1497 "QA: run_qa v1.6" and #1491 codecov bump).

Rebase integration notes:

• The QA group that QA: run_qa v1.6 form + ExplicitImports #1497 added (hand-written if GROUP == "QA"; activate_qa_env(); include("qa/qa.jl")) is now expressed via the run_tests qa kwarg: qa = (; env = joinpath(@__DIR__, "qa"), body = joinpath(@__DIR__, "qa", "qa.jl")). This preserves the exact behavior: QA runs only under GROUP=QA, never as part of All (verified against SciMLTesting 1.1.0 and 1.8.0 — curated all omits "QA", so All runs only the 6 functional groups).
• Dropped the stale test/miscellaneous_tests/explicit_imports.jl include from the Misc group — QA: run_qa v1.6 form + ExplicitImports #1497 deleted that file (its ExplicitImports checks moved into the QA run_qa suite), so the pre-rebase include would have errored the Misc lane. Misc now matches master (only "ReactionSystem Serialisation").
• Root Project.toml compat bumped to SciMLTesting = "1.1" (was "1"): 1.0.0 lacks the all/qa kwargs, so the Downgrade lane would have resolved a version that cannot run run_tests. Both kwargs exist since 1.1.0.

Group dispatch verified locally (dry-run, stubbed bodies) on both the compat floor (1.1.0) and latest (1.8.0): All -> Modeling, Simulation, Hybrid, Misc, Spatial, Extensions; each named group -> itself; QA -> QA; QA excluded from All. All 42 include/env paths resolve to existing files.

Ignore until reviewed by @ChrisRackauckas.

[TorkelE]

Currently trying to fix tests here: #1488. Was done, but then some new breaking things popped up, and then I went travelling and was unable toacess computer for a few days