Quick example : where to draw the line on a unit test

This just came up and I've been asked numerous times "when do I draw the line?" when it comes to a set of tests.  Here's a great example of where that line happens...

 

    [TestClass]
    public class ExportStory_Tests
    {
        private string fakeXml = @"
<rss version=""0.92"">
  <channel>     
    <item>
          <title>[TEST-1] Test</title>
          <type id=""1"" iconUrl=""https://someinstance/"">Story</type>
    </item>
    <item>
          <title>[TEST-2] Test</title>
          <type id=""2"" iconUrl=""https://someinstance/"">Backlog</type>
    </item>
    <item>
          <title>[TEST-3] Test</title>
          <type id=""3"" iconUrl=""https://someinstance/"">Defect</type>
    </item>
    <item>
          <title>[TEST-4] Test</title>
          <type id=""4"" iconUrl=""https://someinstance/"">Task</type>
    </item>
  </channel>
</rss>
";

        private ExportStories _exportStories;
        private IEnumerable<XElement> _result;

        [TestInitialize]
        public void Setup()
        {
            _exportStories = new ExportStories(null, null);
            var elements = XDocument.Parse(fakeStoryXml).XPathSelectElements("rss/channel/item");
            _result = _exportStories.GrabStories(elements, new[] { "story", "backlog" });
        }

        [TestMethod]
        public void grabs_two_stories()
        {
            _result.Count().ShouldEqual(2);
        }

        [TestMethod]
        public void grabs_test_one_and_test_two()
        {
            _result.First().Element("title").Value.ShouldEqual("[TEST-1] Test");
            _result.Last().Element("title").Value.ShouldEqual("[TEST-2] Test");
        }
    }

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