Clean up the documentation for the library testing

[MaximLipnin]

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Issue Details

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Author:	MaximLipnin
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-

[mdh1418]

Can we also add instructions on some guidelines for debugging the tests being ran?

For example using Console.Writeline in various libraries, except System.Private.CoreLib, we need to use Internal.Console.Writeline for iOS/MacCatalyst/tvOS; for Android, we need a few extra steps in order to log these statement lines.

It might also be worth documenting how to use a local xharness build? Would this be the right place to do it? If so, we can put it in the shared general testing README.md in runtime/docs/workflow/testing/libraries.

[akoeplinger]

/cc @lambdageek who was also looking at the docs recently

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Draft Pull Request was automatically closed for inactivity. Please let us know if you'd like to reopen it.

[MaximLipnin]

Let's merge this version for now. If there are any other important details, please let me know and I'll add them to the docs.