Prior to this commit it was possible to configure the
DefaultListableBeanFactory used by the GenericApplicationContext
created by AbstractGenericContextLoader, but it was not possible to
completely replace the bean factory.
This commit introduces a new createContext() factory method in
AbstractGenericContextLoader which indirectly allows subclasses to
supply a custom DefaultListableBeanFactory implementation to the
GenericApplicationContext.
Closes gh-25600
a83529c844