Added DoiDuplicationCheckerTest to increase coverage

[ningxie1991]

This pull request contributes to issue #6207, which is to add more unit tests or improve existing ones. I used Pitest to compute the line and mutation coverage. Since DoiDuplicationChecker did not have its unit test (its method was being tested in other test classes), I added the test to increase both coverages.

Before:

After:

• 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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[koppor]

In general: Looks good to me.

Some code nitpick: variable naming