exitButton.Click += (o, e) =>
{
    int pid = Android.OS.Process.MyPid();
    Android.OS.Process.KillProcess(pid);
};

[Test]
public void ExitButtonClosesTheScreen()
{
    try
    {
        app.Tap(c => c.Button("exitButton"));
        Assert.Fail("App remains open.");
    }
    catch (System.Exception e)
    {
        Assert.AreEqual("The underlying connection was closed: The connection was closed unexpectedly.", e.InnerException.InnerException.InnerException.Message);
    }
}