Add Tests to DoiCleanup

[brunocmo]

Adding unit tests to DoiCleanup class, to improve the coverage of the project.

I added 7 test to the file:

1. cleanupDoientryJustdoi
2. cleanupDoiEntryJustDoiAllEntries
3. cleanupDoiEntryDoiFieldsWithoutHttp
4. cleanupDoiEntryDoiWithSpaces
5. cleanupDoiJustUrl
6. cleanupDoiJustNote
7. cleanupDoiJustEe

• 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, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Thank you for working on tests. Since this also seems to be a coding exercise, some code feedbacks:

Besides small comments, I would suggest to convert all tests checking to a ParameterizedTest. See org.jabref.logic.bst.BibtexCaseChangersTest#testChangeCaseAllLowers for an example. Can be written even more simple in your case --> first paramter the expected entry, the second one the BibEntry to cleanup.

[brunocmo]

Thank you for working on tests. Since this also seems to be a coding exercise, some code feedbacks:

Besides small comments, I would suggest to convert all tests checking to a ParameterizedTest. See org.jabref.logic.bst.BibtexCaseChangersTest#testChangeCaseAllLowers for an example. Can be written even more simple in your case --> first paramter the expected entry, the second one the BibEntry to cleanup.

Thanks for the feedback and tips. I took your sugestion and changed to ParameterizedTest. Still I don't know if the implementation is correctly, if you can spot anything wrong again I would be thankfully.