Only offer agreement download to non-crown for now

We don’t have the crown agreement in a nice downloadable format at the
moment.

tests/app/main/views/test_agreement.py

@@ -15,7 +15,7 @@ class _MockS3Object():
 
 
 @pytest.mark.parametrize('email_address, expected_status', [
-    ('test@cabinet-office.gov.uk', 200),
+    ('test@cabinet-office.gov.uk', 404),
     ('test@aylesburytowncouncil.gov.uk', 200),
     ('test@unknown.gov.uk', 404),
 ])
@@ -36,11 +36,11 @@ def test_show_agreement_page(
 
 
 @pytest.mark.parametrize('email_address, expected_file_fetched, expected_file_served', [
-    (
+    pytest.mark.xfail((
         'test@cabinet-office.gov.uk',
         'crown.pdf',
         'GOV.UK Notify data sharing and financial agreement.pdf',
-    ),
+    ), raises=AssertionError),
     (
         'test@aylesburytowncouncil.gov.uk',
         'non-crown.pdf',