test: model classes of partial evaluator

[lars-reimann]

Summary of Changes

Add missing tests for the model classes of the partial evaluator.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ REPOSITORY	git_diff	yes		no	0.08s
✅ TYPESCRIPT	eslint	8	0	0	5.45s
✅ TYPESCRIPT	prettier	8	0	0	1.28s

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[codecov]

Codecov Report

Merging #704 (a38bd1b) into main (7abd1bd) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #704   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           71        73    +2     
  Lines         9495      9591   +96     
  Branches      1906      1972   +66     
=========================================
+ Hits          9495      9591   +96     

Files	Coverage Δ
packages/safe-ds-lang/src/language/main.ts	100.00% <100.00%> (ø)
...fe-ds-lang/src/language/partialEvaluation/model.ts	100.00% <100.00%> (ø)
...-ds-lang/src/language/typing/safe-ds-core-types.ts	100.00% <100.00%> (ø)
...s-lang/src/language/typing/safe-ds-type-checker.ts	100.00% <100.00%> (ø)
...-lang/src/language/typing/safe-ds-type-computer.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes