Fix some test warnings

[albheim]

Running the test a few times I started to get annoyed at the warnings. Fixed some of the easy ones at least.

[JuliaControlBot]

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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
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❌ 0.042
✔️ 0.006
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[codecov]

Codecov Report

Merging #617 (01547d6) into master (094983a) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
- Coverage   86.81%   86.80%   -0.02%     
==========================================
  Files          31       31              
  Lines        3239     3243       +4     
==========================================
+ Hits         2812     2815       +3     
- Misses        427      428       +1     

Impacted Files	Coverage Δ
src/pid_design.jl	78.48% <0.00%> (-0.09%)	⬇️

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