test: test SCC sorted order

Author: AayushSabharwal

test/scc_nonlinear_problem.jl

@@ -39,6 +39,10 @@ using ModelingToolkit: t_nounits as t, D_nounits as D
         @test prob.u0 isa SVector
         @test !SciMLBase.isinplace(prob)
     end
+
+    # not `issetequal`, this specifically tests that the system is already BLT sorted
+    @test isequal(
+        reduce(vcat, [unknowns(prob.f.sys) for prob in sccprob.probs]), unknowns(model))
 end
 
 @testset "With parameters" begin