Add test for trc if i-gain is zero

[christophfroehlich]

Follow-up to #511 (comment)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.61%. Comparing base (618c961) to head (c89d95e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #526   +/-   ##
=======================================
  Coverage   82.60%   82.61%           
=======================================
  Files          29       29           
  Lines        1983     1984    +1     
  Branches      113      113           
=======================================
+ Hits         1638     1639    +1     
  Misses        276      276           
  Partials       69       69           

Flag	Coverage Δ
unittests	82.61% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
control_toolbox/test/pid_tests.cpp	100.00% <100.00%> (ø)

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