string filename = System.IO.Path.Combine(Directory.GetCurrentDirectory(), "nunit-console.exe");
 Process proc = Process.Start(filename, "/result:" + resultFile + ".xml " + fileName);
proc.WaitForExit();

System.Diagnostics.Process proc;

    private void Btn1Click(object sender, RoutedEventArgs e)
    {
        string filename = System.IO.Path.Combine(Directory.GetCurrentDirectory(), "nunit-console.exe");
        proc = Process.Start(filename, "/result:" + resultFile + ".xml " + fileName);
    }

    private void Btn2Click(object sender, RoutedEventArgs e)
    {
        if (proc != null)
            proc.Kill();
    }