It seems that VS2015 handles solution-level NuGet packages differently (those registered in the
.nuget\packages.config file of the solution). As a result, solution-level NuGet packages must be listed in the projects that use them, otherwise Test Explorer cannot recognise the test runner.
To fix this issue, either re-install the SpecFlow+ Runner NuGet packages, or add the dependency on the
SpecRun.Runner package (
<package id="SpecRun.Runner" version="1.2.0" />) to the packages.config file of your SpecFlow projects. You might need to restart Visual Studio to see your tests.