Reorganize Unitful-related tests to Extensions group

Move all tests that use Unitful functionality from the main test groups
to the Extensions test group, where they belong now that Unitful support
is provided by an extension:

Moved tests:
- units.jl (core Unitful extension tests)
- variable_parsing.jl (uses Unitful units in variable declarations)
- model_parsing.jl (uses Unitful units in model parsing)
- constants.jl (uses Unitful units with constants)

Updated constants.jl to remove UnitfulUnitCheck references:
- Remove UMT = ModelingToolkit.UnitfulUnitCheck
- Change UMT.get_unit(β) to ModelingToolkit.get_unit(β)

These tests now run in the Extensions group where Unitful is available,
ensuring the extension is properly loaded for testing.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

test/constants.jl

@@ -1,7 +1,6 @@
 using ModelingToolkit, OrdinaryDiffEq, Unitful
 using Test
 MT = ModelingToolkit
-UMT = ModelingToolkit.UnitfulUnitCheck
 
 @constants a = 1
 @test isconstant(a)
@@ -25,7 +24,7 @@ simp = mtkcompile(sys)
 
 #Constant with units
 @constants β=1 [unit = u"m/s"]
-UMT.get_unit(β)
+ModelingToolkit.get_unit(β)
 @test MT.isconstant(β)
 @test !MT.istunable(β)
 @independent_variables t [unit = u"s"]

test/runtests.jl

@@ -29,21 +29,18 @@ end
             @safetestset "AbstractSystem Test" include("abstractsystem.jl")
             @safetestset "Variable Scope Tests" include("variable_scope.jl")
             @safetestset "Symbolic Parameters Test" include("symbolic_parameters.jl")
-            @safetestset "Parsing Test" include("variable_parsing.jl")
             @safetestset "Simplify Test" include("simplify.jl")
             @safetestset "Direct Usage Test" include("direct.jl")
             @safetestset "System Linearity Test" include("linearity.jl")
             @safetestset "Input Output Test" include("input_output_handling.jl")
             @safetestset "Clock Test" include("clock.jl")
             @safetestset "ODESystem Test" include("odesystem.jl")
             @safetestset "Dynamic Quantities Test" include("dq_units.jl")
-            @safetestset "Unitful Quantities Test" include("units.jl")
             @safetestset "Mass Matrix Test" include("mass_matrix.jl")
             @safetestset "Reduction Test" include("reduction.jl")
             @safetestset "Split Parameters Test" include("split_parameters.jl")
             @safetestset "StaticArrays Test" include("static_arrays.jl")
             @safetestset "Components Test" include("components.jl")
-            @safetestset "Model Parsing Test" include("model_parsing.jl")
             @safetestset "Error Handling" include("error_handling.jl")
             @safetestset "StructuralTransformations" include("structural_transformation/runtests.jl")
             @safetestset "Basic transformations" include("basic_transformations.jl")
@@ -58,7 +55,6 @@ end
             @safetestset "DAE Jacobians Test" include("dae_jacobian.jl")
             @safetestset "Jacobian Sparsity" include("jacobiansparsity.jl")
             @safetestset "Modelingtoolkitize Test" include("modelingtoolkitize.jl")
-            @safetestset "Constants Test" include("constants.jl")
             @safetestset "Parameter Dependency Test" include("parameter_dependencies.jl")
             @safetestset "Equation Type Accessors Test" include("equation_type_accessors.jl")
             @safetestset "System Accessor Functions Test" include("accessor_functions.jl")
@@ -141,5 +137,9 @@ end
         @safetestset "BifurcationKit Extension Test" include("extensions/bifurcationkit.jl")
         @safetestset "InfiniteOpt Extension Test" include("extensions/test_infiniteopt.jl")
         @safetestset "Auto Differentiation Test" include("extensions/ad.jl")
+        @safetestset "Unitful Extension Test" include("units.jl")
+        @safetestset "Variable Parsing with Units Test" include("variable_parsing.jl")
+        @safetestset "Model Parsing with Units Test" include("model_parsing.jl")
+        @safetestset "Constants with Units Test" include("constants.jl")
     end
 end