fix: explicitly export the items in the falcon.testing namespace

[jap]

... so mypy also likes it when falcon.testing.TestClient is used

Summary of Changes

Replace this text with a high-level summary of the changes included in this PR.

Related Issues

#2387

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[jap]

(unsure whether there should be tests for this and how to implement them if necessary)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4d04e9b) to head (a8e1e6a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2388   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7725      7726    +1     
  Branches      1071      1071           
=========================================
+ Hits          7725      7726    +1     

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[vytas7]

Well spotted, we were trying to add __all__s everywhere, but apparently missed this file. Thanks!