Add @GuiTest annotation

[koppor]

Fix

org.jabref.gui.fieldeditors.LinkedFileViewModelTest
  Test downloadHtmlWhenLinkedFilePointsToHtml() FAILED
  java.lang.RuntimeException: java.lang.UnsupportedOperationException: Unable to open DISPLAY
  Test downloadPdfFileWhenLinkedFilePointsToPdfUrl() FAILED
  java.lang.RuntimeException: java.lang.UnsupportedOperationException: Unable to open DISPLAY

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.

[github-actions]

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

[Siedlerchr]

Fixes https://github.com/JabRef/jabref-issue-melting-pot/issues/390