As documented in the Detecting Test Configuration section of the Spring Boot reference manual, any beans configured in a top-level class annotated with @TestConfiguration will not be picked up via component scanning. So you have to explicitly register your @TestConfiguration class.
You can do that either via @Import(MyTestConfiguration.class) or @ContextConfiguration(classes = MyTestConfiguration.class) on your test class.
On the other hand, if your class annotated with @TestConfiguration were a static nested class within your test class, it would be registered automatically.