Add arch axis and group alias to root test-group matrix

[ChrisRackauckas-Claude]

Summary

Adds two new root-matrix-only per-group keys to test/test_groups.toml (consumed by grouped-tests.yml), so a package can declare a 32-bit (or any non-native arch) test lane through the matrix instead of bypassing it with a hand-added tests.yml job:

• arch — string or list of Julia CPU architectures. The group runs once per arch; empty (the default) means the runner's native arch. Emitted as a per-cell arch field that grouped-tests.yml forwards to tests.yml's existing julia-arch input (empty string → tests.yml falls back to runner.arch).
• group — the GROUP env value dispatched to the package's runtests.jl, defaulting to the section name. This lets a section named e.g. "Core 32-bit" dispatch the "Core" body — so its group folder / core file resolves under SciMLTesting folder-discovery mode — while carrying its own distinct arch/os axis and its own CI job name.

grouped-tests.yml now forwards julia-arch: ${{ matrix.arch }} and appends the arch to the job name when non-empty (e.g. Core (julia 1, ubuntu-latest, x86)).

Motivating example

[Core]
versions = ["1", "lts", "pre"]
os = ["ubuntu-latest", "macos-latest", "windows-latest"]

["Core 32-bit"]
group = "Core"
versions = ["1"]
os = ["ubuntu-latest"]
arch = "x86"

[QA]
versions = ["lts", "1"]

yields the 9 native Core cells + QA exactly as before, plus a single Core (julia 1, ubuntu-latest, x86) cell dispatching GROUP=Core.

Backward compatibility

Existing test_groups.toml files are unaffected: no arch/group keys means arch is "" (native) and the section name is the dispatched group — identical output to before this change.

Testing

• Extended test/runtests.jl with three new testsets covering the arch axis (single value + list fan-out), the group alias (32-bit lane dispatching GROUP=Core), and the --root-matrix CLI emitting the arch field.
• Full julia test/runtests.jl suite passes locally (exit 0).
• Both changed .jl files are Runic-clean.

Follow-up

This is the enabling half of a two-PR change. The consumer PR (SciML/FindFirstFunctions.jl) adopts a "Core 32-bit" section to test 32-bit platforms, replacing a hand-added tests-32bit job.

Please ignore until reviewed by @ChrisRackauckas.

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