Introduce MSTest lifecycle documentation

[marcelwgn]

Summary

This PR introduces documentation for the MSTest test lifecycle as discussed here: microsoft/testfx#5637

---

Internal previews

📄 File	🔗 Preview link
docs/core/testing/unit-testing-mstest-writing-tests-lifecycle.md	MSTest lifecycle
docs/navigate/devops-testing/toc.yml	docs/navigate/devops-testing/toc

[marcelwgn]

Open questions:

• Is the formatting fine?
• Are the author fields filled out correctly?
• What else need to be configured for this page to correctly show up in the navigation

[Youssef1313]

What else need to be configured for this page to correctly show up in the navigation

You need to update toc.yml for that.

docs/docs/navigate/devops-testing/toc.yml

Lines 84 to 101 in ef844cc

	- name: Writing tests
	items:
	- name: Overview
	href: ../../core/testing/unit-testing-mstest-writing-tests.md
	- name: Examples
	items:
	- name: C# example
	href: ../../core/testing/unit-testing-csharp-with-mstest.md
	- name: F# example
	href: ../../core/testing/unit-testing-fsharp-with-mstest.md
	- name: VB example
	href: ../../core/testing/unit-testing-visual-basic-with-mstest.md
	- name: Attributes
	href: ../../core/testing/unit-testing-mstest-writing-tests-attributes.md
	- name: Assertions
	href: ../../core/testing/unit-testing-mstest-writing-tests-assertions.md
	- name: TestContext
	href: ../../core/testing/unit-testing-mstest-writing-tests-testcontext.md

[Youssef1313]

LGTM

[BillWagner]

Thanks @marcelwgn

This LGTM as well, and I'll now.