[fix][test] Fix thread leaks in tests by closing executors properly

[lhotari]

Motivation

There are thread leaks in tests caused by executors (thread pools) that aren't shutdown properly.

Modifications

Properly shutdown executors that have been created during test execution.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.27%. Comparing base (a0f8b0d) to head (e8c2599).
Report is 1387 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #21425       +/-   ##
=============================================
+ Coverage     34.55%   73.27%   +38.71%     
- Complexity    12083    32573    +20490     
=============================================
  Files          1711     1888      +177     
  Lines        130708   140307     +9599     
  Branches      14245    15420     +1175     
=============================================
+ Hits          45172   102811    +57639     
+ Misses        79503    29410    -50093     
- Partials       6033     8086     +2053     

Flag	Coverage Δ
inttests	24.18% <ø> (?)
systests	24.76% <ø> (-0.03%)	⬇️
unittests	72.55% <ø> (+40.68%)	⬆️

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see 1470 files with indirect coverage changes

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