Factor out `cc_test_so()` helper function

[danielocfb]

Introduce the cc_test_so() helper function in order to get rid of all the arguments necessary for compiling this test binary.

[codecov[bot]]

Codecov Report

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

Project coverage is 94.60%. Comparing base (47e6bf7) to head (0c9c106).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #687   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files          48       48           
  Lines        9612     9612           
=======================================
  Hits         9093     9093           
  Misses        519      519           

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