Use SciMLTesting v1.2 (folder-based run_tests)

[ChrisRackauckas-Claude]

Converts the test suite to the SciMLTesting v1.2 folder-based run_tests() model.

test/runtests.jl is now just:

using SciMLTesting
run_tests()

Group routing is derived from the folder layout + test/test_groups.toml (unchanged):

• Core = top-level test/*.jl (explicit_imports.jl + recipes.jl, the latter extracted from the old inline plotting smoke test in runtests.jl).
• QA = test/qa/ with its existing per-group Project.toml sub-env (Aqua + JET).
• All = Core (the only non-QA group), matching the previous GROUP == "All" || GROUP == "Core" behavior.

Deps: added SciMLTesting + SafeTestsets to the root [extras]/[targets].test/[compat] and to test/qa/Project.toml. test/test_groups.toml is unchanged. Per-GROUP file sets verified identical to master via folder discovery.

This supersedes the earlier v1.1.0 explicit-args rollout on this branch.

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Updated to SciMLTesting v1.1.0 with the curated all = ["Core"] kwarg.

The first pass left all unset, so run_tests used the legacy fallback (core + every env-less group + qa), which ran QA under the bare/All group. But the master test/runtests.jl All/default branch ran only the Core body — QA was a separate if GROUP == "QA" branch never reached by All. The corrected call:

run_tests(;
    core = joinpath(@__DIR__, "core.jl"),
    qa = (; body = joinpath(@__DIR__, "qa", "qa.jl"), env = joinpath(@__DIR__, "qa")),
    all = ["Core"],
)

makes All/unset run exactly Core (QA omitted), Core run Core, and QA run the QA body — identical to master for every GROUP value. Behavior-equivalent refactor; no test-assertion or logic changes. Static-verified (TOML.parsefile + Meta.parseall on all test files).

Ignore until reviewed by @ChrisRackauckas.