REF: Fix PET model exception test functions

[jhlegarreta]

Fix PET model exception test functions:

• Refactor functions so that the last midpoint value exception is properly captured: rename tests functions so that their purposes becomes apparent from the name and add a test to check the last midpoint value exception.
• Check both midframe and xlim missing exceptions.
• Define the exception messages so that tests can check exactly the captured message.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.20%. Comparing base (09df9e8) to head (2f6fa20).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
+ Coverage   77.01%   77.20%   +0.18%     
==========================================
  Files          28       28              
  Lines        1775     1781       +6     
  Branches      177      177              
==========================================
+ Hits         1367     1375       +8     
+ Misses        348      347       -1     
+ Partials       60       59       -1     

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