I added the "xUnit.net runner for Visual Studio" v0.99.8 via Extensions Manager, but when I open the Test Explorer window, it does not seem to pick up any of my unit tests. Also, the Resharper 9 EAP does which is the only version of Resharper that supports VS2015 does seem yet to have the plugin for xUnit Test Runner.
How then, can I run xUnit Unit Tests in VS2015 Preview?

Regarding managing / running the tests through test explorer, it is available through nuget. I had to install package xunit.runner.visualstudio version 0.99.9-build1021 in the test projects. After building the solution, the tests showed up fine and I was able to run them.
BTW, this was for a windows service but should work for ASP.NET/Web projects.