xfail pydap datatree tests (#10869)

* xfail pydap datatree tests

* fix xfail

* xfail variant

* alternative xfails

* alternative xfails

---------

Co-authored-by: Kai Mühlbauer <kmuehlbauer@wradlib.org>

Author: shoyer

xarray/tests/test_backends_datatree.py

@@ -569,6 +569,8 @@ def test_roundtrip_using_filelike_object(self, tmpdir, simple_datatree) -> None:
 class TestPyDAPDatatreeIO:
     """Test PyDAP backend for DataTree."""
 
+    pytestmark = pytest.mark.xfail(reason="test.opendap.org reports a 404 error")
+
     engine: T_DataTreeNetcdfEngine | None = "pydap"
     # you can check these by adding a .dmr to urls, and replacing dap4 with http
     unaligned_datatree_url = (