add quantization test

Author: baggepinnen

docs/make.jl

@@ -1,4 +1,5 @@
 using ModelingToolkitSampledData
+using ModelingToolkit
 using Documenter
 using Plots
 gr(fmt=:png)

docs/src/tutorials/noise.md

@@ -102,6 +102,7 @@ A signal may be quantized to a fixed number of levels (e.g., 8-bit) using the [`
 ```@example QUANT
 using ModelingToolkit, ModelingToolkitSampledData, OrdinaryDiffEq, Plots
 using ModelingToolkit: t_nounits as t, D_nounits as D
+using ModelingToolkitStandardLibrary.Blocks
 z = ShiftIndex(Clock(0.1))
 @mtkmodel QuantizationModel begin
     @components begin

test/test_discrete_blocks.jl

@@ -457,7 +457,8 @@ end
     sol = solve(prob, Tsit5(), dtmax=0.01)
     y = sol[m.quant.y]
     uy = unique(y)
-    @test length(uy) == 4
+    @test length(uy) <= 4
+    @test 0 ∈ uy
 end