Support for compiling and running generated code in tests

[snicoll]

With the AOT engine now generating an optimized view of the bean factory, there is a need to be able to compile and run the generated code in a test to assert that it has the intended effect.

Such general purpose utility could land in a new spring-core-test module for a start (with, potentially, a dependency on spring-core).

[philwebb]

I've pushed a branch here for review. Currently the package is under org.springframework.aot.test.generator.