added testing documentation

[csc530]

contributes to #1410

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

I added pages three new pages to the docs related to the testing namespace of Spectre Console.
It gives examples and some usage and use cases for the testing classes (that are hopefully correct and reflect reality).
The documentation is written from what I could find, understand, and have done using Spectre Console.

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Please upvote 👍 this pull request if you are interested in it.

[csc530]

@microsoft-github-policy-service agree