[fcanas]

Factoring out a common constructor in tests is an example of making the tests resilient against changes in the underlying code. If the constructor changes, the tests need to be fixed in one place, not in 50.

Other examples may be around a `setup` method. If the method is private, don't test it. Then you can refactor freely. If it's public, test the pre/post conditions around the method in as few places as possible (hopefully one). Even if other tests rely on the object having been "setup", just trust that it works. If the specification of `setup` changes, you just have the `setup` tests to update, not the entire object.