Decouple from `UserDefaults.standard` in tests

A recent issue in the tests and @dvdchr's workaround for it make it clear that we have some state leaking into UserDefaults.standard and affecting the unit tests.

This ought not to happen. There are various ways to tackle this problem. For example:

• Inject an in-memory UserDefaults instead of accessing .standard.
• Use Dependency Inversion, defining one or more protocols to place between domain object and UserDefaults
• Dup and restore the UserDefaults state before and after each test that interacts with it

Shameless plug, see Test-Driven Development in Swift, Chapter 15.

Before picking one, we should spend some time inspecting the codebase for UserDefaults accesses. I wouldn't be surprised if more than one approach will be necessary, depending on the complexity level of different consumers.