Allocation tests

[DanielDoehring]

Related to #1664

After addressing #1679 by #1684 there are no unreasonable high allocations of the custom time integrators.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e900262) 82.10% compared to head (c34a98d) 82.10%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1687   +/-   ##
=======================================
  Coverage   82.10%   82.10%           
=======================================
  Files         425      425           
  Lines       34382    34382           
=======================================
  Hits        28229    28229           
  Misses       6153     6153           

Flag	Coverage Δ
unittests	82.10% <ø> (ø)

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[ranocha]

Thanks!