Add tests for MaterialisedEnumerationObserver

[claucambra]

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 92.77108% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.40%. Comparing base (9326f6b) to head (1b8ed67).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
Tests/Interface/MockEnumerator.swift	81.48%	5 Missing ⚠️
.../Enumeration/MaterialisedEnumerationObserver.swift	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   77.72%   78.40%   +0.67%     
==========================================
  Files          42       44       +2     
  Lines        9850     9932      +82     
==========================================
+ Hits         7656     7787     +131     
+ Misses       2194     2145      -49     

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