Add test and format

Author: YingboMa

src/systems/abstractsystem.jl

@@ -163,7 +163,9 @@ independent_variables(sys::AbstractTimeIndependentSystem) = []
 independent_variables(sys::AbstractMultivariateSystem) = getfield(sys, :ivs)
 
 iscomplete(sys::AbstractSystem) = isdefined(sys, :complete) && getfield(sys, :complete)
-complete(sys::AbstractSystem) = isdefined(sys, :complete) ? (@set! sys.complete = true) : sys
+function complete(sys::AbstractSystem)
+    isdefined(sys, :complete) ? (@set! sys.complete = true) : sys
+end
 
 for prop in [:eqs
              :noiseeqs

test/components.jl

@@ -32,6 +32,8 @@ end
 
 include("../examples/rc_model.jl")
 
+completed_rc_model = complete(rc_model)
+@test isequal(completed_rc_model.resistor.n.i, resistor.n.i)
 @test ModelingToolkit.n_extra_equations(capacitor) == 2
 @test length(equations(structural_simplify(rc_model, allow_parameter = false))) == 2
 sys = structural_simplify(rc_model)