Add test for SVG fallback image path
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

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin