Combine tests for central org email branding

Central orgs have more options than others, including the option
to revert back to GOV.UK once branding is set. Combining the tests
together should make that a bit clearer.

tests/app/utils/test_branding.py

@@ -58,57 +58,39 @@ def test_get_available_choices_service_assigned_to_org(
     assert list(options) == expected_options
 
 
-@pytest.mark.parametrize('branding_type, expected_options', [
-    ('email', [
+@pytest.mark.parametrize('service_branding, expected_options', [
+    (None, [
         ('govuk_and_org', 'GOV.UK and Test Organisation'),
         ('organisation', 'Test Organisation'),
     ]),
-    ('letter', [
+    ('1234-abcd', [
+        ('govuk', 'GOV.UK'),
+        ('govuk_and_org', 'GOV.UK and Test Organisation'),
         ('organisation', 'Test Organisation'),
     ])
 ])
-def test_get_available_choices_with_central_org(
+def test_get_available_choices_email_branding_central_org(
     mocker,
     service_one,
-    branding_type,
+    service_branding,
     expected_options,
     mock_get_service_organisation,
-):
-    service = Service(service_one)
-
-    mocker.patch(
-        'app.organisations_client.get_organisation',
-        return_value=organisation_json(organisation_type='central'),
-    )
-
-    options = get_available_choices(service, branding_type=branding_type)
-    assert list(options) == expected_options
-
-
-def test_get_available_choices_email_branding_set(
-    mocker,
-    service_one,
-    mock_get_service_organisation,
     mock_get_email_branding,
 ):
     service = Service(service_one)
 
     mocker.patch(
         'app.organisations_client.get_organisation',
-        return_value=organisation_json()
+        return_value=organisation_json(organisation_type='central'),
     )
     mocker.patch(
         'app.models.service.Service.email_branding_id',
         new_callable=PropertyMock,
-        return_value='1234-abcd',
+        return_value=service_branding,
     )
 
     options = get_available_choices(service, branding_type='email')
-    assert list(options) == [
-        ('govuk', 'GOV.UK'),
-        ('govuk_and_org', 'GOV.UK and Test Organisation'),
-        ('organisation', 'Test Organisation'),
-    ]
+    assert list(options) == expected_options
 
 
 def test_get_available_choices_letter_branding_set(