To test this I created brand new WP8 project. "Program" is as simple as they get, using default MainPage and all.

I just added MainViewModel, having only one property
public string MyProperty
{
get { return _myProperty; }
set { _myProperty = value; OnPropertyChanged(); }
}
After that I added the WP8 Unit Test project and replaced default UnitTest1.Testmethod1 with
[TestClass]
public class MainViewModelTests
{
[TestMethod]
public void MyPropertyTest()
{
string expected = "expected";
var vm = new MainViewModel();
vm.MyProperty = expected;
Assert.AreEqual(expected, vm.MyProperty);
}
}
Both MS Test runner and ReSharper are working fine and test passed.

I am using Visual Studio 2012 v11.0.61030.00 Update 4 and JetBrains ReSharper v7.1 build 7.1.3000.2259.
If interested you can find the Visual Studio solution from GitHub @ https://github.com/mikkoviitala/wp8-unit-test-example and try it out yourself. If it's not working for you then I can't think of anything other than reinstalling ReSharper and hope for the best.