This is hard to test as no plagiarism plugins exist in core, and plagiarism plugins that support forum submission all require subscriptions. Configure a plagiarism plugin to submit forum post content to the service (not all plagiarism plugins support this), submit some text and make sure the link for the external system still displays. this patch has already been tested using the Turnitin and Urkund plugins - Dan may be able to supply test credentials for the tester during integration, but I suspect in this case a sanity check of the code would be sufficient.