test: Fix storage_client fixture cleanup

[janbuchar]

• fixes a bunch of random "Event loop is closed" warnings in tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (8b53e27) to head (564e07f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1841      +/-   ##
==========================================
- Coverage   92.27%   92.24%   -0.03%     
==========================================
  Files         157      157              
  Lines       10909    10909              
==========================================
- Hits        10066    10063       -3     
- Misses        843      846       +3     

Flag	Coverage Δ
unit	92.24% <ø> (-0.03%)	⬇️

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[vdusek]

nice, thanks