[mdatagen] add env support for tests section

[atoulme]

Description

Allow to set fixed env vars for generated tests.

Testing

Added unit tests and tested with generation of code across all components of the repository.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.57%. Comparing base (57f059b) to head (8a9dfbc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12692   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files         482      482           
  Lines       26377    26377           
=======================================
  Hits        24154    24154           
  Misses       1762     1762           
  Partials      461      461           

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