TST: xfail messages with reference

 to the corresponding issues

unittests/test_data_model.py

@@ -25,7 +25,7 @@ class DataModelTest(unittest.TestCase):
         assert "testproperty" in names
 
     # TODO this seems to require integration test
-    @pytest.mark.xfail
+    @pytest.mark.xfail(reason="This test will need an integration test. See issue #51")
     def test_get_existing_entities(self):
         db.RecordType(name="TestRecord").insert()
         c = db.Container().extend([

unittests/test_table_importer.py

@@ -64,7 +64,6 @@ class ConverterTest(unittest.TestCase):
             r"\this\computer,\this\computer"),
                          ["/this/computer", "/this/computer"])
 
-    @pytest.mark.xfail
     def test_datetime(self):
         test_file = os.path.join(os.path.dirname(__file__), "date.xlsx")
         self.importer = XLSImporter(converters={'d': datetime_converter,
@@ -165,7 +164,7 @@ class XLSImporterTest(unittest.TestCase):
         df_new = self.importer.check_unique(df)
         self.assertEqual(df_new.shape[0], 1)
 
-    @pytest.mark.xfail
+    @pytest.mark.xfail(reason="Wrong Error is raised. See Issue #52")
     def test_raise(self):
         tmp = NamedTemporaryFile(delete=False, suffix=".lol")
         tmp.close()