compat: declare SciMLTesting for SciCompDSL

[ChrisRackauckas-Claude]

This PR declares SciMLTesting for the SciCompDSL subpackage test environment and includes it in the test target so the test dependency is explicit under the stricter SciMLTesting 2.4+ QA setup.

Ignore until reviewed by @ChrisRackauckas.

Verification:

$ timeout 3600 julia --project=lib/SciCompDSL -e 'using Pkg; Pkg.test()'
SCICOMPDsl_TEST_EXIT=0
Test Summary:           | Pass  Broken  Total     Time
Model parsing - MTKBase |  200       1    201  3m40.5s
Test Summary:       | Pass  Total     Time
Model parsing - MTK |  205    205  1m26.6s
Testing SciCompDSL tests passed

$ git diff --check origin/master...HEAD
# passed

$ typos $(git diff --name-only origin/master...HEAD)
# passed

Not verified here: full root ModelingToolkit test matrix and downstream jobs.

[ChrisRackauckas-Claude]

Clean-base reproduction completed at origin/master (efcce552e2d1c088382f9d3b5da5da03632ceb23).

Direct control for this compat-only PR:

/home/crackauc/.juliaup/bin/julia +lts --startup-file=no --project=lib/SciCompDSL -e 'using Pkg; Pkg.test()'

Result:

Model parsing - MTKBase | 200 pass, 1 broken, 201 total
Model parsing - MTK | 205 pass, 205 total
Testing SciCompDSL tests passed

The clean root QA reproduction also exits with 50 passed, 0 failed, 2 errored after 34m06s. The only errors are SciMLTesting no_implicit_imports and no_stale_explicit_imports, both because ExplicitImports cannot analyze the dynamic include in src/precompile.jl; the same root QA failures are therefore pre-existing on origin/master, not caused by this PR's SciCompDSL compat declaration.

[ChrisRackauckas-Claude]

Clean-master investigation confirms this PR's only change is the SciCompDSL SciMLTesting compat entry; it does not cause the red lanes. Clean master reproduces the same failures: QA JET cannot analyze dynamically included precompile code; docs have pre-existing duplicate/missing entries and unresolved references; Julia pre InterfaceI fails in the existing Symbolics change-of-variables path; downgrade fails in the existing OrdinaryDiffEqCore/DelayDiffEq limiter compatibility layer. Bisect and CI links are recorded in the investigation report. No tests were weakened.