Split out existing tests related to submitting branding

The existing tests were parameterized to contain the cases both where
the branding form is successfully submitted and those where it isn't. We
were using `pytest.mark.xfail` to check that the tests fail as expected
if there were errors on the form. However, since I'll be changing how
the form validation works, I want to make sure that these tests were
actually failing because of the form validation and not because of
another reason, such as a slight difference in Zendesk ticket output.

This creates separate tests for cases where data entered in the form
is invalid.

tests/app/main/views/test_service_settings.py

@@ -5064,18 +5064,6 @@ def test_show_branding_request_page_when_branding_is_same_as_org(
         },
         'Something else\n\nHomer Simpson'
     ),
-    pytest.param(
-        {
-            'options': 'something_else',
-        },
-        '[Missing details]',
-        marks=pytest.mark.xfail(raises=AssertionError),
-    ),
-    pytest.param(
-        {'options': 'foo'},
-        'Nope',
-        marks=pytest.mark.xfail(raises=AssertionError),
-    ),
 ))
 @pytest.mark.parametrize('org_name, expected_organisation', (
     (None, 'Can’t tell (domain is user.gov.uk)'),
@@ -5147,27 +5135,28 @@ def test_submit_email_branding_request(
     )
 
 
-@pytest.mark.parametrize('data, requested_branding', (
-    (
-        {
-            'options': 'something_else',
-            'something_else': 'Homer Simpson'
-        },
-        'Something else\n\nHomer Simpson'
-    ),
-    pytest.param(
-        {
-            'options': 'something_else',
-        },
-        '[Missing details]',
-        marks=pytest.mark.xfail(raises=AssertionError),
-    ),
-    pytest.param(
-        {'options': 'foo'},
-        'Nope',
-        marks=pytest.mark.xfail(raises=AssertionError),
-    ),
+@pytest.mark.parametrize('data, error_message', (
+    ({'options': 'something_else'}, 'Cannot be empty'),  # no data in 'something_else' textbox
+    ({'options': 'foo'}, 'Select an option'),  # no radio button selected
 ))
+def test_submit_email_branding_request_when_form_has_missing_data(
+    client_request,
+    service_one,
+    data,
+    error_message,
+    mock_get_email_branding,
+):
+    service_one['email_branding'] = sample_uuid()
+
+    page = client_request.post(
+        '.email_branding_request', service_id=SERVICE_ONE_ID,
+        _data=data,
+        _follow_redirects=True,
+    )
+    assert page.h1.text == 'Change email branding'
+    assert normalize_spaces(page.select_one('.error-message').text) == error_message
+
+
 @pytest.mark.parametrize('org_name, expected_organisation', (
     (None, 'Can’t tell (domain is user.gov.uk)'),
     ('Test Organisation', 'Test Organisation'),
@@ -5176,8 +5165,6 @@ def test_submit_letter_branding_request(
     client_request,
     service_one,
     mocker,
-    data,
-    requested_branding,
     mock_get_service_settings_page_common,
     mock_get_letter_branding_by_id,
     no_reply_to_email_addresses,
@@ -5207,7 +5194,10 @@ def test_submit_letter_branding_request(
 
     page = client_request.post(
         '.letter_branding_request', service_id=SERVICE_ONE_ID,
-        _data=data,
+        _data={
+            'options': 'something_else',
+            'something_else': 'Homer Simpson',
+        },
         _follow_redirects=True,
     )
 
@@ -5220,8 +5210,8 @@ def test_submit_letter_branding_request(
             '',
             '---',
             'Current branding: HM Government',
-            'Branding requested: {}\n',
-        ]).format(expected_organisation, requested_branding),
+            'Branding requested: Something else\n\nHomer Simpson\n',
+        ]).format(expected_organisation),
         subject='Letter branding request - service one',
         ticket_type='question',
         user_name='Test User',
@@ -5237,6 +5227,35 @@ def test_submit_letter_branding_request(
     )
 
 
+@pytest.mark.parametrize('data, error_message', (
+    ({'options': 'something_else'}, 'Cannot be empty'),  # no data in 'something_else' textbox
+    ({'options': ''}, 'Select an option'),  # no radio button selected
+))
+def test_submit_letter_branding_request_when_form_has_missing_data(
+    client_request,
+    mocker,
+    service_one,
+    organisation_one,
+    data,
+    error_message,
+    mock_get_letter_branding_by_id,
+):
+    mocker.patch(
+        'app.organisations_client.get_organisation',
+        return_value=organisation_one,
+    )
+    service_one['letter_branding'] = sample_uuid()
+    service_one['organisation'] = organisation_one
+
+    page = client_request.post(
+        '.letter_branding_request', service_id=SERVICE_ONE_ID,
+        _data=data,
+        _follow_redirects=True,
+    )
+    assert page.h1.text == 'Change letter branding'
+    assert normalize_spaces(page.select_one('.error-message').text) == error_message
+
+
 @pytest.mark.parametrize('from_template', [
     None,
     TEMPLATE_ONE_ID