Test persistency

[akiradeveloper]

fix included:

• reaper should be terminated when the wrapping Backend is dropped.

other than this, huge modifications to the test code.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.86%. Comparing base (d595d2f) to head (e972d63).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #432      +/-   ##
==========================================
+ Coverage   93.94%   94.86%   +0.92%     
==========================================
  Files          47       47              
  Lines        2426     2434       +8     
==========================================
+ Hits         2279     2309      +30     
+ Misses        147      125      -22     

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