[NOTIFICATION] Agent plugin test tool has been updated

[Sheng Wu <wu...@apache.org>]

Hi Dev team

As PR 4456[1] has been approved and merged into the main repo, the agent
plugin tool is no longer hosted in the main repo, instead, it has its own
repo[2]. During this change, our PMC, Haochao Zhuang, updated the source
codes of the tool to adopt the all 6.x core concept from old 5.x names. So
the expected YAML has a little incompatible change. Documents have been
updated, *if you are writing a new plugin with tests, please take a look to
avoid facing test failure*.

*NOTICE, the plugin tool is not going to have plan to do ASF release. *Other
repos feel free to use git clone with required commit id. We hope in this
way making the test tool update and bug fix easier, as no one besides our
dev team(in or out ASF repo) will use this tool.

[1] https://github.com/apache/skywalking/pull/4458
[2] https://github.com/apache/skywalking-agent-test-tool

Sheng Wu 吴晟

Apache SkyWalking
Apache Incubator
Apache ShardingSphere, ECharts, DolphinScheduler podlings
Zipkin
Twitter, wusheng1108