integration tests: make randomize_username() the default

[bnjbvr]

Most tests would randomize the username when creating a TestClientBuilder; make it the default, since it's a sensible choice, and avoids interference between different tests / test runs.

A single test required an actual non-randomized username, so a specific way to opt out from this new default behavior has been introduced.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.17%. Comparing base (fbeb77a) to head (f9fca67). Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3641      +/-   ##
==========================================
- Coverage   84.19%   84.17%   -0.02%     
==========================================
  Files         256      256              
  Lines       26594    26594              
==========================================
- Hits        22390    22385       -5     
- Misses       4204     4209       +5     

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