test: add missing using Test to runtests.jl (fixes @testset UndefVarError)

[ChrisRackauckas-Claude]

Problem

test/runtests.jl opens with using Pkg but never imports Test. It then calls @testset/@test_throws at the top level (the "Invalid usage error path" block), so the test process dies with:

UndefVarError: `@testset` not defined

This is a test-harness scaffolding bug introduced by the grouped-tests conversion (the top-level @testset was added to runtests.jl itself, which unlike core.jl has no using Test). It breaks the Core, downgrade, and All-group CI.

Fix

Add the missing using Test immediately after using Pkg so Test is in scope before the top-level @testset. This is the minimal change — one import line. No test assertions or logic were touched. (core.jl already has its own using Test.)

Verification

Ran locally on Julia 1.11:

GROUP=Core julia +1.11 --project=. -e 'using Pkg; Pkg.test()'

Result — all groups pass, including the previously-failing top-level testset:

Test Summary: | Pass  Total  Time
Unit tests    |   23     23  1.7s
End-to-end tests |   11     11
Issue #3      |    1      1
Commutation with other Macros |    4      4
Invalid usage error path |    1      1   <-- was the @testset UndefVarError
     Testing ConcreteStructs tests passed

---

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas]

Superseded by the SciMLTesting v1.2 migration (folder-based run_tests()). Master already carries this test as test/invalid_usage_tests.jl, and the missing-using Test issue no longer applies under run_tests(). Closing as already-fixed.