add a number of missing tests

[baggepinnen]

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[JuliaControlBot]

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Plots were compared to references. 8/11 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
❌ 0.048
✔️ 0.0
✔️ 0.001
✔️ 0.001
✔️ 0.0
❌ 0.043
❌ 0.059
✔️ 0.01

[codecov]

Codecov Report

Merging #707 (c916f76) into master (53a1864) will increase coverage by 0.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #707      +/-   ##
==========================================
+ Coverage   87.92%   88.43%   +0.51%     
==========================================
  Files          35       35              
  Lines        3628     3624       -4     
==========================================
+ Hits         3190     3205      +15     
+ Misses        438      419      -19     

Impacted Files	Coverage Δ
src/types/SisoTfTypes/SisoRational.jl	93.67% <ø> (+14.15%)	⬆️
src/types/StateSpace.jl	90.82% <0.00%> (+3.38%)	⬆️

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