[Python] add docs about how to run tests locally (or even simplify it if it's too complex)

[kezhenxu94]

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Description

Related to apache/skywalking-python#214 (review) .

Our contributor has to look around the repo for demo scripts to run the tests locally. Sometimes they have to try over and over. I'd expect the guide to run tests locally should be clearly documented step by step.

We should add a new section in https://github.com/apache/skywalking-python/blob/master/docs/en/contribution/PluginTest.md

Use case

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Related issues

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Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[jiang1997]

I write a guide to local test in this commit. Do you have any suggestions for this before I pull request.

[wu-sheng]

You could send a pull request directly. It is fine if more polish could be done.

[kezhenxu94]

I write a guide to local test in this commit. Do you have any suggestions for this before I pull request.

Feel free to just open a pull request. You don't have to make it perfect before opening a pull request.

[jiang1997]

Got it.

[kezhenxu94]

Closed by apache/skywalking-python#222