How to unit test with IHttpClientFactory

Suggest to add a section on how to write unit tests for components using the IHttpClientFactory to create your HttpClient. I don't see it done easily, as you still get the HttpClient which can't be mocked without accessing protected members.

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* ID: 881cef77-11ef-ac11-06d0-0996a1ccfa1b
* Version Independent ID: 3d356334-e672-7e8b-1fef-7944ebee99eb
* Content: [Make HTTP requests using IHttpClientFactory in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-6.0#use-ihttpclientfactory-in-a-console-app)
* Content Source: [aspnetcore/fundamentals/http-requests.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/fundamentals/http-requests.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-fundamentals**
* GitHub Login: @stevejgordon
* Microsoft Alias: **scaddie**