How to avoid overusing interfaces when unit testing

When unit testing, you usually just want to mock classes that make external calls (be it a database query or hitting an API). Consequently, you need to have an interface for these classes. However, a lot of times you may have random DTOs or utility classes that do small simple things and don't need to be mocked.

Some languages have tools that let you mock classes without writing a corresponding interface; for instance, Python allows patching arbitrary classes in test code, and the Mockito library for Java can generate a mock object from any class.

There is a school of thought that says when writing unit tests, only the "leaves", i.e. classes and methods which don't invoke other classes within the application, should be unit tested in isolation, and classes that are involved in orchestrating the behavior of other classes should be tested at the acceptance or integration level. This style of testing avoids writing unit tests that depend on mocking most of the classes in your application, and so avoids the need to write interfaces. Martin Fowler's article on mocks covers some of the differences between different styles of testing regarding mocks.

In order to write effective tests, you do need seams in your code (a place that let's you break your dependencies apart) to allow you to control any dependencies in your code. Interfaces are possibly the most obvious way to do this, but you can use other techniques such as wrapping your dependency in a method on your class under test and declaring that method as protected virtual (c#) and overriding the class for your unit tests. There is a great series of videos on YouTube around TDD that deal with design decisions when doing TDD. Search for "is TDD dead".