Add TESTed-DSL publication (with preprint)

[pdawyndt]

Since we have no preprint for the submitted TESTed-DSL publication, we should add a self-hosted preprint of TESTed-DSL paper and the reference to it on the publications page (it is also allowed to self-host preprints).

[pdawyndt]

Related: there's an issue with the DOI-link of the "Mining patterns in syntax trees to automate code reviews of student solutions for programming exercise" paper. The DOI-link appears twice on the page, and both are linked to the Dodona-publications page itself.