The ArcGIS Runtime does not support running under .NET Core. "Only" .NET Framework 4.6.1+, .NET Native with UWP or Mono on Android/iOS is supported.
The install path property you're referring to is specific to .NET Framework. I'm guessing because you're trying to target .NET Core, it's pulling in the .NET Standard library during compilation, which only contains common members across all of the above. It is however a compile-time only reference assembly and isn't using during execution (we're using bait'n'switch to execute with the correct ArcGIS Runtime Assembly). It's only there to allow you to compile .NET Standard class libraries that references the ArcGIS runtime. However your app project heads(which your unit test project is one of) must be one of the above mentioned targets.