Organized the contents of the test suite

[fktn-k]

This PR has organized the contents of the test suite, for example, the naming convention of the unit test cases or renaming of existing unit test cases whose name was too ambiguous to grasp what is tested.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions[bot]]

:octocat: Upload Coverage Event Notification

Coverage data has been uploaded for the commit 80a8094c5c17714629bf4368814cc19746d30434. You can download the artifact which contains the same file uploaded to the Coveralls and its HTML version.

Name	fkYAML_coverage.pr311.zip
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URL	https://github.com/fktn-k/fkYAML/actions/runs/8741380379/artifacts/1426623704