I'm unit testing a class which has two dependencies, both of which are external packages (I'm using C# so these are NuGet packages, AutoMapper and CompareNETObjects).
(NOTE: Both are just utilities, related to converting an object from one type to another, and to comparing objects. Neither makes any external calls)
Should I mock these external packages or is it acceptable to assume they are robust building blocks that don't need testing, similar to the .NET libraries?
Or, perhaps, should I decide whether to test each package on a case-by-case basis? Say don't test popular open source packages as we could assume they're well tested but do test less popular ones that don't have as many users or contributers.
I know answers to this question are likely to be opinions but I'd like to find out what experienced unit-testers think.