SpringBoot Relax ImportsContextCustomizer to rely only on a test class name

ImportsContextCustomizer currently creates an infrastructure that requires a reference to a test Class but it turns out that only the FQN of the test class is required.

We can relax this private arrangement to only use the name. A side effect of that is that it would prevent https://github.com/spring-projects/spring-framework/issues/29168 from happening.

Comment From: snicoll

If we decided to not include ImportsContextCustomizer at runtime, then no bean definition would be contributed and that would make this issue irrelevant. However, given that the runtime effectively only need a class name, it makes sense to change this regardless.