Overhaul GroovyBeanDefinitionReaderTests

This commit switches from CGLIB-based proxies to JDK dynamic proxies
for AOP and scoped-bean tests, in order to be able to run tests in the
Eclipse IDE without special Run Configuration setup for the Java module
system.

This commit also simplifies the loading of beans defined with the Groovy
DSL.

See gh-27945

Author: sbrannen