Add tests

Author: YingboMa

test/odesystem.jl

@@ -756,3 +756,23 @@ let
     @test arr == sol
     @test typeof(arr) == typeof(sol)
 end
+
+
+let
+    @parameters t
+
+    u = collect(first(@variables u[1:4](t)))
+    Dt = Differential(t)
+
+    eqs = [
+           Differential(t)(u[2]) - 1.1u[1] ~ 0
+           Differential(t)(u[3]) - 1.1u[2] ~ 0
+           u[1] ~ 0.0
+           u[4] ~ 0.0
+          ]
+
+    ps = []
+
+    @named sys = ODESystem(eqs, t, u, ps)
+    @test_nowarn simpsys = structural_simplify(sys)
+end