Use SciMLTesting v1.2 folder-based run_tests

Convert ModelingToolkitStandardLibrary.jl to the SciMLTesting v1.2
folder-discovery model. runtests.jl becomes `using SciMLTesting;
run_tests()`.

Layout:
- Core: the functional test files are flattened from their domain
  subfolders (Blocks/, Electrical/, Thermal/, Magnetic/, Mechanical/,
  Hydraulic/) to the top level of test/, which is the Core group in the
  folder model. Folder-discovery does not recurse into subfolders, so
  Core must live at the top level. Basenames do not collide.
- QA: test/aqua.jl moves to test/qa/aqua.jl. QA runs in the main test
  env (Aqua is in the root [extras]); no test/qa/Project.toml is added.
- test/Mechanical/multibody.jl moves to test/shared/ because it is
  present on disk but is NOT run by the old runtests.jl (it depends on
  CairoMakie). test/shared/ is not a declared group, so it is never
  discovered, preserving that exclusion.

Per-GROUP behavior is preserved exactly: GROUP=Core runs the same 19
functional files as before (multibody excluded); GROUP=QA runs Aqua.

SciMLTesting added to [extras], [targets].test, and [compat] ("1").

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -36,6 +36,7 @@ SafeTestsets = "0.1"
 SciCompDSL = "1"
 SciMLBase = "2, 3"
 SciMLStructures = "1.7.0"
+SciMLTesting = "1"
 SymbolicIndexingInterface = "0.3.39"
 Symbolics = "7.4.1"
 Test = "1"
@@ -60,8 +61,9 @@ SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
 SciCompDSL = "91a8cdf1-4ca6-467b-a780-87fda3fff15e"
 SciMLBase = "0bca4576-84f4-4d90-8ffe-ffa030f20462"
 SciMLStructures = "53ae85a6-f571-4167-b2af-e1d143709226"
+SciMLTesting = "09d9d899-5365-40a9-917a-5f67fddea283"
 SymbolicIndexingInterface = "2efcf032-c050-4f8e-a9bb-153293bab1f5"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["ADTypes", "Aqua", "LinearAlgebra", "OrdinaryDiffEqBDF", "OrdinaryDiffEqDefault", "OrdinaryDiffEq", "OrdinaryDiffEqNonlinearSolve", "OrdinaryDiffEqRosenbrock", "OrdinaryDiffEqSDIRK", "SafeTestsets", "Test", "ControlSystemsBase", "DataFrames", "DataInterpolations", "SciMLBase", "SciMLStructures", "SymbolicIndexingInterface", "ForwardDiff", "SciCompDSL", "ModelingToolkit"]
+test = ["ADTypes", "Aqua", "LinearAlgebra", "OrdinaryDiffEqBDF", "OrdinaryDiffEqDefault", "OrdinaryDiffEq", "OrdinaryDiffEqNonlinearSolve", "OrdinaryDiffEqRosenbrock", "OrdinaryDiffEqSDIRK", "SafeTestsets", "SciMLTesting", "Test", "ControlSystemsBase", "DataFrames", "DataInterpolations", "SciMLBase", "SciMLStructures", "SymbolicIndexingInterface", "ForwardDiff", "SciCompDSL", "ModelingToolkit"]