Add test for SVG fallback image path

It's the only resource that uses the `asset_path`
template variable in `template.njk` so this checks
that works as expected.

From feedback on the associated pull request:

https://github.com/alphagov/notifications-admin/pull/3241#issuecomment-571102394

tests/app/main/views/test_index.py

@@ -177,6 +177,15 @@ def test_css_is_served_from_correct_path(client_request):
         ][index])
 
 
+def test_resources_that_use_asset_path_variable_have_correct_path(client_request):
+
+    page = client_request.get('main.documentation')  # easy static page
+
+    logo_svg_fallback = page.select_one('.govuk-header__logotype-crown-fallback-image')
+
+    assert logo_svg_fallback['src'].startswith('https://static.example.com/images/govuk-logotype-crown.png')
+
+
 @pytest.mark.parametrize('extra_args, email_branding_retrieved', (
     (
         {},