Use a test runner that can handle the pumper's stop Deferred

[exarkun]

Fixes #422

[codecov[bot]]

Codecov Report

Merging #423 (a3be713) into main (02e4acf) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
- Coverage   96.38%   96.37%   -0.02%     
==========================================
  Files          63       63              
  Lines        7525     7526       +1     
  Branches     1012     1012              
==========================================
  Hits         7253     7253              
  Misses        202      202              
- Partials       70       71       +1     

Impacted Files	Coverage Δ
src/_zkapauthorizer/tests/test_client_resource.py	98.22% <100.00%> (+0.22%)	:arrow_up:
src/_zkapauthorizer/tahoe.py	93.35% <0.00%> (-0.74%)	:arrow_down:

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