Add Swift 5.3 beta in the tests

[vinivendra]

The Swift 5.3 release branch already exists, which means we can start testing with it to catch any errors early and be ready for when it launches. It might be a good idea to mark it as a development branch in the tests, so that we can check how recent it is and complain if it gets too old (forcing devs to keep it updated, otherwise there's no point in testing it at all).