In agile like development who should write test cases

Question

Our team has a task system where we post small incremental tasks assigned to each developer.

Each task is developed in its own branch, and then each branch is tested before being merged to the trunk.

My question is: Once the task is done, who should define the test cases that should be done on this task?

Ideally I think the developer of the task himself is best suited for the job, but I have had a lot of resistance from developers who think it's a waste of their time, or that they simply don't like doing it.

The reason I don't like having my QA people do it, is because I don't like the idea of them creating their own work. For example they might leave out things that are simply too much work to test, and they may not know the technical detail that is needed.

But likewise, the down part of developers doing the test cases, is that they may leave out things that they think will break. (even subconsciously maybe)

As the project manager, I ended up writing the test cases for each task myself, but my time is taxed and I want to change this.

Suggestions?

EDIT: By test cases I mean the description of the individual QA tasks that should be done to the branch before it should be merged to the trunk. (Black Box)

Answer 1

Basically, test as if you were an end user, completing the tasks that end users should be able to complete, as specified in the requirements. The QA team and product managers should write these test cases because they (supposedly) know what the customer wants and how they should utilise the application.