Set xfail to longstanding failing test

This is for code that is run to backfill data on new deployments.

See: #354

tests/test_backfill.py

@@ -72,6 +72,7 @@ def backfill_test_data(db):
     )
 
 
+@pytest.mark.xfail  # See: https://github.com/appsembler/figures/issues/354
 def test_backfill_monthly_metrics_for_site(backfill_test_data):
     """Simple coverage and data validation check for the function under test