tests: fix spurious failure of stats tests

[efahl]

Some of the test_stats tests would fail occasionally with the wrong queue length. Traced it back to the redis 'Queue' object somehow persisting across tests. We now ensure that a fresh queue is used for each test.

[efahl]

Second push includes the changes I made to test_stats.py to chase this down, specifically adds assertions to ensure that the queues are empty before doing anything else.

Also incorporates a bunch of cleanup to make the test cases much more readable.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 90.93%. Comparing base (5e65dec) to head (08d4698). :warning: Report is 281 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1051       +/-   ##
===========================================
+ Coverage   80.75%   90.93%   +10.17%     
===========================================
  Files          15       14        -1     
  Lines         977     1147      +170     
===========================================
+ Hits          789     1043      +254     
+ Misses        188      104       -84     

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