Fix world age issues in test suite

[fingolfin]

This fixes warnings that turn up in the tests due to the world age changes.

I am afraid my changes are rather made with a sledge hammer. If someone sees a more elegant solution, I'd be happy to learn about it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.65%. Comparing base (c3fc036) to head (4d870bc).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files           2        2           
  Lines         177      177           
=======================================
  Hits          164      164           
  Misses         13       13           

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