Define the independent variable in the motor test (#483)

* Define the motor test independent variable

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

* Import independent variable macro from its owner

Depend directly on ModelingToolkitBase public API instead of receiving the macro through ModelingToolkit re-export.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

---------

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>

Author: ChrisRackauckas-Claude

test/motor.jl

@@ -1,9 +1,12 @@
 using ModelingToolkit, Test
+using ModelingToolkitBase: @independent_variables
 using SciCompDSL
 using SciMLBase
 using ModelingToolkitStandardLibrary.Thermal
 using ModelingToolkitStandardLibrary.Blocks
 
+@independent_variables t
+
 # https://doc.modelica.org/Modelica%204.0.0/Resources/helpWSM/Modelica/Modelica.Thermal.HeatTransfer.Examples.Motor.html
 
 @testset "Thermal Motor Demo" begin