How to get Selenium tests to run after deployment in visual studio team services

I am having an issue getting selenium end to end tests to work after an automated deployment using visual studio team services (VSTS).

I have a build working that generates a build artefact. This is triggered from VSTS but runs on an on premises build server. I have a deployment working that deploys to an on premises development web server. All this works including unit tests running after the build.

When I try to add testing after the deployment is when I run into the problem. The tests are to be run on the build server and point to the dev server website. The deployment has two phases. A deploy and then an agent phase that runs a test assemblies task using the build agent on the build server. The problem seems to be that the test dll's are not being included in the build artifact and so are never found when the test process runs. Deploy setup us as follows.

I have a copy files before the publish artifact in the build definition that seems to copy the files in to the right place but they are not included in the zip file artefact. I've looked at several websites and posts on here but I still seem to be missing a vital bit of knowledge that will get this working.