Add documentation about the TestContext

[Evangelink]

Fixes microsoft/testfx#4040

---

Internal previews

📄 File	🔗 Preview link
docs/core/testing/unit-testing-mstest-writing-tests-testcontext.md	The TestContext class
docs/core/testing/unit-testing-mstest-writing-tests.md	Write tests with MSTest
docs/navigate/devops-testing/toc.yml	docs/navigate/devops-testing/toc

[gewarren]

Thanks for adding this. It would be ideal if you can add the longer code snippets to a project so it's automatically verified to compile.

[Evangelink]

It would be ideal if you can add the longer code snippets to a project so it's automatically verified to compile.

Could you please provide me with an example?

[gewarren]

Could you please provide me with an example?

Yes, here is an example that links to a separate code file that's part of a project. You can either reference an entire code file, or a specific "snippet" as defined by "id". You can also highlight certain lines in the snippet using "highlight". Example.

[Evangelink]

Ping @IEvangelist

[Evangelink]

@gewarren Could you please review/merge?