There's a partial example at https://github.com/aspnet/HttpClientFactory/wiki/Consumption-Patterns#unit-testing. This content needs to be added to Docs.
See the xUnit & Moq sample at https://github.com/glennc/httpclientfactorysample/tree/master/test/WebUI.Tests.
FYI: The first url redirects to https://docs.microsoft.com/nl-nl/aspnet/core/fundamentals/http-requests?view=aspnetcore-2.1
@jphellemons Thanks for pointing this out. All of the content on that Wiki page, except for the unit testing, was moved to Docs.
@scottaddie the unit testing part was exactly what I was looking for.
@jphellemons Here's a screenshot of what that Wiki page used to contain for unit testing:

See also the link above to the xUnit & Moq sample.
@guardrex please suggest a doc this issue should be attached to.
@Rick-Anderson Perhaps none because we only keep two unit tests topics (Razor Pages tests & Test controllers), and this doesn't look like it fits neatly into either of them. I think Scott's plan was to make a new topic for this under the Test, debug, and troubleshoot node.
This should be a new topic.
@scottaddie we don't have the resources to maintain our current testing docs, so I'm closing this issue.