[chore] Move testonly code into a separate internal package

[bogdandrutu]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.16%. Comparing base (216046a) to head (82e4965).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12494   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files         465      466    +1     
  Lines       25201    25201           
=======================================
  Hits        23226    23226           
  Misses       1576     1576           
  Partials      399      399           

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