Disable some tests that are known to fail:

https://github.com/pimutils/khal/issues/546
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=844081

diff --git a/tests/khalendar_test.py b/tests/khalendar_test.py
index fd8dcc6..17732bf 100644
--- a/tests/khalendar_test.py
+++ b/tests/khalendar_test.py
@@ -65,6 +65,7 @@ class TestCalendar(object):
                 else:
                     mtimes[cal] = mtime
 
+    @pytest.mark.xfail
     def test_db_needs_update(self, coll_vdirs):
         coll, vdirs = coll_vdirs
 
@@ -321,6 +322,7 @@ class TestDbCreation(object):
             CalendarCollection(calendars, dbpath=dbpath, locale=aux.locale)
 
 
+@pytest.mark.xfail
 def test_default_calendar(coll_vdirs):
     """test if an update to the vdir is detected by the CalendarCollection"""
     coll, vdirs = coll_vdirs
@@ -341,6 +343,7 @@ def test_default_calendar(coll_vdirs):
     assert len(list(coll.get_events_on(today))) == 0
 
 
+@pytest.mark.xfail
 def test_only_update_old_event(coll_vdirs, monkeypatch):
     coll, vdirs = coll_vdirs