Add Sendable conformance to the TestingSetup type

[tib]

This PR adds Sendable conformance to the TestingSetup type in the HummingbirdTesting target.

[adam-fowler]

Looks good, as long as everything compiles we're good

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.95%. Comparing base (12aed8f) to head (224082e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #783      +/-   ##
==========================================
+ Coverage   82.94%   82.95%   +0.01%     
==========================================
  Files         124      124              
  Lines        6185     6185              
==========================================
+ Hits         5130     5131       +1     
+ Misses       1055     1054       -1     

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