Visual Studio 2017 15.5 Preview 2 introduced a new real time test discovery feature. This feature is live with 15.6.
When using VS 2017 15.6 with SpecFlow+ Runner, the Test Explorer will not display any tests in VS unless you enable Additionally discover tests from built assemblies after builds under Tools | Options | Test:
We are looking into how we can support real time test discovery with SpecFlow+ Runner. Until then, please make sure you enable this option in VS if you are using SpecFlow+ Runner with Visual Studio 2017 15.6.