total rewrite & container issue: unit test ASP.NET Core apps

This topic:

* Has high traffic and the lowest CSAT (Customer Satisfaction) of any high traffic ASP.NET Core doc
* Is highly opinionated are far more complicated than necessary. See #15809
* Only shows controllers with views. RP and API skipped.
* Does not show how to set up unit tests
* Is far to complex

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This is the container issue for this doc. Reopen each issue you are working on so the owner is notified.

PU issue
- [ ] Inject dummy controller? #19358
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Not PU:
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Total rewrite outline:

* Sample app will be a `IsPrime()` code used in [Unit testing C# in .NET Core using dotnet test and xUnit](https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-with-dotnet-test). The endpoints will be implemented in Razor Pages, MVC controller with views, and Web API
* rewrite will be similar to [Unit testing C# in .NET Core using dotnet test and xUnit](https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-with-dotnet-test).
* How to set up a unit test soln (app + test project)
* Basic unit tests
* use xUnit and Moq

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#### Document Details

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* ID: b5cce372-24e2-3489-cb19-c85d934e5d3b
* Version Independent ID: 9d12b19f-a94e-4df4-357b-3f3b93696eb6
* Content: [Test controller logic in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/testing?view=aspnetcore-3.0)
* Content Source: [aspnetcore/mvc/controllers/testing.md](https://github.com/aspnet/AspNetCore.Docs/blob/master/aspnetcore/mvc/controllers/testing.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-mvc**
* GitHub Login: @ardalis
* Microsoft Alias: **riande**