Split ObjectAssertionSpecs in multiple files

[vbreuss]

Split the huge ObjectAssertionSpecs file into multiple partial classes, similar to the change in #1903 for StringAssertionSpecs.

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR adds a feature or fixes a bug, please update the release notes with a functional description that explains what the change means to consumers of this library, which are published on the website.
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• If the PR affects the documentation, please include your changes in this pull request so the documentation will appear on the website.
  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome

[github-actions]

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It seems all right 👌

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[coveralls]

Pull Request Test Coverage Report for Build 6683878417

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.388%

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Totals
Change from base Build 6663997299:	0.0%
Covered Lines:	11713
Relevant Lines:	11904

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💛 - Coveralls

[jnyrup]

For reference: this work started with the discussion in #1551.
We didn't create any guidelines back then on when we considered a test class of nested classes eligible to be split into separate files.

Some shell scripts I made while looking into this:
Longest file lengths:

find . -name "*.cs" -exec wc -l {} + | head -n -1 | sort -n | grep -vEi '\.[a-z]+\.cs'

Most nested classes:

grep -rEc --include=*.cs '^    public class' | grep -vEi '(\.[a-z]+\.cs|:[01]$)' | sort -t':' -k 2 -n

[dennisdoomen]

Nice!