Allow third-parties to contribute ContextCustomizers that can customize
ApplicationContexts created by the Spring TestContext Framework (TCF)
before they are refreshed.
A customizer may be provided via a ContextCustomizerFactory which is
registered with `spring.factories`. Each factory is consulted whenever
a new ApplicationContext needs to be created by the TCF. Factories may
inspect various details about the test and either return a new
ContextCustomizer or null.
ContextCustomizers are similar to ApplicationContextInitializers and
may perform any number of tasks, including bean registration, setting
of active profiles, etc.
Issue: SPR-13998
87a3a5cb3b