add unit test to generateInstitutionKey

[leaf-soba]

• prepare to refactor this method

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.

[koppor]

Thank you for working on this.

Small nitpick on the test itself.

[koppor]

Please do a separate test for null - this "hack" is only needed for one out of the n>10 test cases. --> The implementation of the test method should be "straight-forward" / Trivial

[leaf-soba]

OK I'll fix it, thanks for reviewing it.

[koppor]

@leaf-soba Thank you for using software engineering techniques here. In case you want to have your local files containing all modifications you have, but you want to split up the pull requests to smaller items/topics, I recommend GitButler.