Add dotnet test --collect:"Code Coverage" instructions

[kendrahavens]

We should mention either the dotnet code coverage cli option or Coverlet.

This link: https://github.com/Microsoft/vstest-docs/blob/master/docs/analyze.md#working-with-code-coverage does not cover the dotnet test --collect"Code Coverage"

Related to #17256

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Document Details

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• ID: a64f7e75-fbea-18ba-f290-3e5bb3ab0da7
• Version Independent ID: 24639c20-6c3a-738e-2ff8-9a6ee6f232dc
• Content: Unit testing in .NET Core and .NET Standard - .NET Core
• Content Source: docs/core/testing/index.md
• Product: dotnet-core
• GitHub Login: @ardalis
• Microsoft Alias: wiwagn