Debugging a R# test runner is, thankfully, not an experience most people will have to go through.
Because ReSharper runs unit tests in a separate process, spun up on demand, you need to find some way to attach to this process.
The brute force approach is simply to throw an exception somewhere in your plugin, and then attach.
A better alternative is to launch VS in ReSharper.Internal mode (or “god mode”, as some like to refer to it) by supplying the /ReSharper.Internal argument to devenv.exe. (You can also enable a specific plugin by using /ReSharper.Plugin “MyPlugin.dll”, which spares some of the pain when trying to build a plugin that is in use).
This provides you with some extra options. The one you want is “Enable Debug (Internal)” which can be found in ReSharper->Options->Tools->Unit Testing:
Once this is enabled, when you run a test a dialog will appear:
Giving you time to attach to the test runner: