Ensure `JdkAopProxyTests` and `CglibAopProxyTests` are actually run in the Gradle build

[sbrannen]

Overview

The abstract ProxyExceptionHandlingTests class introduced in #32469 has a rather odd structure of inheritance and @Nested test classes which results in the static nested JdkAopProxyTests and CglibAopProxyTests classes not being run (or at least reported correctly) in our Gradle build.

We should therefore rework those classes to ensure the tests are actually run.

Related issues

• Consistent handling of undeclared checked exceptions in CGLIB proxies #32469