Fixes #14134.

The Problem

The ClassGeneratorTest.testMain test is non-idempotent and fails upon repeated execution within the same JVM instance due to self-induced state pollution. Specifically, the test employs the ClassGenerator to configure a custom class with a predetermined name (Bean.class.getName() + "$Builder"). Subsequently, the test invokes ClassGenerator.toClass() to instantiate the desired Class<?> object. However, within ClassGenerator.toClass(), the invoked method javassist.ClassPool.makeClass() is called to create the class. Given that Javassist freezes a class upon its initial loading (ensuring classes cannot be altered or removed at runtime), repeated invocations of testMain() fail as the class name remains unchanged, leading to class generation errors.

Sample Failure Message (in the 2nd run of the test):

java.lang.RuntimeException: org.apache.dubbo.common.bytecode.Bean$Builder: frozen class (cannot edit)
	at javassist.ClassPool.checkNotFrozen(ClassPool.java:595)
	at javassist.ClassPool.makeClass(ClassPool.java:837)
	at javassist.ClassPool.makeClass(ClassPool.java:814)
	at org.apache.dubbo.common.bytecode.ClassGenerator.toClass(ClassGenerator.java:318)
	at org.apache.dubbo.common.bytecode.ClassGenerator.toClass(ClassGenerator.java:302)
	at org.apache.dubbo.common.bytecode.ClassGeneratorTest.testMain(ClassGeneratorTest.java:197)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

What is the purpose of the change

Fixing this issue is recommended since unit tests should be idempotent (deterministically passing in repeated runs).

Brief changelog

Assign a unique ID to each class to be generated in the same classloader.

Verifying this change

After the patch, running the test multiple times within the same JVM will not lead to failures.

Additional Notes

There are some other non-idempotent tests detected in the test suite. I can open a new PR to address the others if you find this fix reasonable.

Checklist

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