How to update the testrunner if multiple peoples are working in the same project while commit & pull

If those are all in the same project, I don't think you can rely on the merge in the repository to help out.

You have a couple of choices.  If it was my project and all those test cases are related, I would have one person work on them all.  If you need separate people to work on them, then you could give each of them a separate project to work on and then merge them into one project and commit that project. Once they are merged, anyone who works on those tests should start from that project.