Unit & Integration Testing utilities

Unit & Integration testing ADK related code could perhaps benefit from some simple common shared utilities?

This isn't about some sort of full blown "evaluation" toolkit, but just along the lines of _"I want to be able to have to write less code to write a simple (integration) test for something like the https://google.github.io/adk-docs/get-started/quickstart/#create-multitoolagentjava"._

[`core/src/test/java/com/google/adk`](https://github.com/google/adk-java/tree/main/core/src/test/java/com/google/adk) has some utility classes a bit like that. But they are not part of a published Maven artifact, and intended for testing the ADK itself, not for its users.

The [`RunLoop`](https://github.com/google/adk-java/blob/d2042949dc22115294f2fd21433c8b1387044b9b/contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/RunLoop.java#L29) which is used in the [`LangChain4jIntegrationTest`](https://github.com/google/adk-java/blob/d2042949dc22115294f2fd21433c8b1387044b9b/contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jIntegrationTest.java#L74) (from @glaforge) is a similar kind of basically generic utility.

I have **started to create more like that [here](https://github.com/enola-dev/enola/tree/800a6496fbdc66e64b7dbe6f15042e7026558a8a/java/dev/enola/ai/adk/test),** which can be used e.g. like _[this](https://github.com/enola-dev/enola/blob/200f065096abc6c6df9bccdeec349661a8bbe5d4/java/dev/enola/ai/adk/core/QuickstartDemoTest.java#L45)._

Is there perhaps any interest in accepting contributions for "upstreaming" (a subset of) that into this repo?

Or maybe I should publish my own "ADK testing library", and it could be linked to?

I'm interested in feedback and thoughts from other ADK users about this.