Defer creating the system under test in properties

[Baptouuuu]

This is in preparation for #80

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (3ead210) to head (2d7f99a).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #87      +/-   ##
=============================================
+ Coverage      91.58%   91.60%   +0.02%     
+ Complexity      1745     1743       -2     
=============================================
  Files            163      163              
  Lines           9372     9361      -11     
=============================================
- Hits            8583     8575       -8     
+ Misses           789      786       -3     

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