Relax ImportsContextCustomizer to rely only on a test class name

[snicoll]

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 spring-projects/spring-framework#29168 from happening.

[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.