ci: improve test coverage and add snapshot tests for object conditions

[alexanderlinne]

With this PR we add new test cases for methods defined in ArchUnitNET/Fluent/Syntax/Elements/ObjectsShould.cs and ArchUnitNET/Fluent/Syntax/Elements/ObjectConditionsDefinition.cs that achieves the following:

• We achieve full line coverage and nearly full branch coverage by testing a larger subset of the interface and more combinations of arguments
• We allow for the detection of regressions in the error messages by adding snapshot tests
• The new tests have an improved execution time compared to the older tests, because we do not use the whole ArchUnitNETTests assembly as the architecture for our tests. Instead, we add separate assemblies that contain a small number of types. This reduces the number of hits per line from hundreds or thousands to usually less than ten.

The new tests will replace tests in ArchUnitNETTests/Fluent/Syntax/Elements/ObjectSyntaxElementsTests.cs, but to remove these tests without reducing the code coverage, additional tests for ArchUnitNET/Fluent/Syntax/Elements/ObjectPredicatesDefinition.cs will be needed.

Relates to #229, providing a way to check for changes to the error messages in pull requests

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.37%. Comparing base (0ebfbdf) to head (a428ef6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
+ Coverage   56.70%   65.37%   +8.66%     
==========================================
  Files         254      254              
  Lines       22842    22842              
  Branches     1938     1938              
==========================================
+ Hits        12953    14932    +1979     
+ Misses       9334     7368    -1966     
+ Partials      555      542      -13     

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