Here's a solution that doesn't require changing anything, as it will generate an assembly named App_GlobalResources.dll with all the resources embedded, the way the tests expect.
Just call it from a method marked with the AssemblyInitialize attribute and it will run only once, before all tests start:
public static void GenerateResourceAssembly()
{
    var testExecutionFolder = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
    var solutionRootPath = "PATH_TO_YOUR_SOLUTION_ROOT";
    //Somewhere similar to C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin
    var pathResgen = "PATH_TO_RESGEN.EXE"; 
    //You may need to adjust to the path where your global resources are
    var globalResourcesPath = Path.Combine(solutionRootPath, @"Web\App_GlobalResources");
    var parameters = new CompilerParameters
    {
        GenerateExecutable = false,
        OutputAssembly = "App_GlobalResources.dll"
    };
    foreach (var pathResx in Directory.EnumerateFiles(globalResourcesPath, "*.resx"))
    {
        var resxFileInfo = new FileInfo(pathResx);
        var filename = resxFileInfo.Name.Replace(".resx", ".resources");
        var pathResources = Path.Combine(testExecutionFolder, "Resources." + filename);
        var startInfo = new ProcessStartInfo
        {
            CreateNoWindow = true,
            WindowStyle = ProcessWindowStyle.Hidden,
            FileName = Path.Combine(pathResgen, "resgen.exe"),
            Arguments = string.Format("\"{0}\" \"{1}\"", pathResx, pathResources)
        };
        using (var resgen = Process.Start(startInfo))
        {
            resgen.WaitForExit();
        }
        parameters.EmbeddedResources.Add(pathResources);
    }
    CodeDomProvider.CreateProvider("CSharp").CompileAssemblyFromSource(parameters);
}