This is a bit of fiddly solution, but you could configure some external tools for each of group of tests you want to run. I'm not sure if you'll be able to launch the ReSharper test runner this way, but you can run the console version of nunit. Once you have of those tools setup, you can assigned keyboard shortcuts to the commands "Tools.ExternalCommand1", "Tools.ExternalCommand2", etc.
This wont really scale very well, and it's awkward to change - but it will give you keyboard shortcuts for running your tests. It does feel like there should be a much simpler way of doing this.