Add tests for discrete feedback

[baggepinnen]

closes #304

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 9/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
✔️ 0.003
✔️ 0.003
✔️ 0.0
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.042
✔️ 0.006
✔️ 0.011

[codecov]

Codecov Report

Merging #626 (1f6e9aa) into master (9455a54) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #626   +/-   ##
=======================================
  Coverage   86.75%   86.75%           
=======================================
  Files          31       31           
  Lines        3253     3253           
=======================================
  Hits         2822     2822           
  Misses        431      431           

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