Add extraction test

Author: HarrisonGrodin

test/system_construction.jl

@@ -91,6 +91,8 @@ eqs = [_x ~ y/C,
        D(x) ~ -A*x,
        D(y) ~ A*x - B*_x]
 de = DiffEqSystem(eqs,[t],[x,y],Variable[_x],[A,B,C])
+de2 = DiffEqSystem(eqs,[t])
+test_vars_extraction(de, de2)
 @test eval(ModelingToolkit.generate_ode_function(de))([0.0,0.0],[1.0,2.0],[1,2,3],0.0) ≈ -1/3
 
 # Now nonlinear system with only variables