Add tests for mass import

[solth]

This pull request adds unit, integration and Selenium tests for mass import.

Part of https://github.com/orgs/kitodo/projects/8

[BartChris]

Tests cover all existing functionality and look good to me.
One could maybe think of an integration tests doing a full test wether a uploaded CSV with two proceses actually creates those two processes. (the ImportService itself is already tested) But this could be added later.

[stweil]

My change request was implemented – thank you.