Fix asyncio tests

[simonw]

I'm going to keep hacking on this until it works.

• Test failures in CI: There is no current event loop in thread 'MainThread' #2461

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📚 Documentation preview 📚: https://datasette--2464.org.readthedocs.build/en/2464/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.36%. Comparing base (1902735) to head (8bcd3a7).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2464      +/-   ##
==========================================
- Coverage   92.55%   90.36%   -2.20%     
==========================================
  Files          41       41              
  Lines        6438     6469      +31     
==========================================
- Hits         5959     5846     -113     
- Misses        479      623     +144     

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