How and where you define the test cases, and the resulting test plan, is at your own discretion; there are many tools available.
Limited to tracking the test case and its results. This ensures that maintenance remains straightforward, and the document can provide a clear overview of the testing progress.
Maintained as a single copy, available to all appropriate members of the project team.
Neutral and limited to the test results. It is the responsibility of the project manager to make decisions on any actions resulting from the test results.