Add tests under Mill 0.12.x

[lefou]

• Update mill-integrationtest to 0.7.2
• Added Test running plugin with Mill 0.12.0 and 0.12.14

Fix: #212

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.75%. Comparing base (9043805) to head (c0126fb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files           5        5           
  Lines          48       48           
  Branches        2        2           
=======================================
  Hits           45       45           
  Misses          3        3           

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