test: use repr instead of display

Author: franckgaga

test/1_test_sim_model.jl

@@ -285,9 +285,8 @@ end
     @test linmodel1.Bd ≈ linmodel2.Bd
     @test linmodel1.C  ≈ linmodel2.C
     @test linmodel1.Dd ≈ linmodel2.Dd 
-
-    display(nonlinmodel1.linbuffer)
-    display(nonlinmodel1.linbuffer.buffer_f_at_u_d)
+    @test repr(nonlinmodel1.linbuffer) == "LinearizationBuffer object"
+    @test repr(nonlinmodel1.linbuffer.buffer_f_at_u_d) == "DifferentiationBuffer with a JacobianConfig"
 
     f1!(ẋ, x, u, d, _) = (ẋ .= x.^5 + u.^4 + d.^3; nothing)
     h1!(y, x, d, _) = (y .= x.^2 + d; nothing)

test/4_test_plot_sim.jl

@@ -2,7 +2,8 @@
     using .SetupMPCtests, ControlSystemsBase, LinearAlgebra
     model = LinModel(sys, Ts, i_d=[3])
     res = sim!(model, 15)
-    display(res)
+
+    @test repr(res) == "Simulation results of LinModel with 15 time steps."
     @test isa(res.obj, LinModel)
     @test length(res.T_data) == 15
     @test res.U_data[:, 1] ≈ model.uop .+ 1