Add explicit tests on Mac and Windows

[koppor]

Triggered by #9988

This PR enables running our tests on Windows and Mac OS. In case we add some OS-specific funtionality with tests, they will be executed on the CI, too.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

cli tests with string align fails. Seems probably an issue with the whitespace char or so

[Siedlerchr]

Some FileUtil Test fails on Windows and on mac the css file base64 encoding differs. Guess this is okay.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.

[Siedlerchr]

Closing this due to inactivity and currenltly not that important