NUnit .NET Core running through ReSharper
ReSharper is known to have problems running .NET Core 2.0 tests in Visual Studio 2017. Try to run them without ReSharper.
For more information see this Stack Overflow question and answers: .NET Core 2.0 and xUnit doesn't run.
I solved by installing the following NuGet packages
- NUnit
- NUnit3TestAdapter
- Microsoft.NET.Test.Sdk