closes https://github.com/TryGhost/Ghost/issues/13920
- Custom excerpt should be used as a fallback for meta_description in line with the behaviour of OG and Twitter metadata
- We specifically don't want to use the full fallback to the auto-generated preview text when a custom excerpt isn't defined, because we trust search engines to be able to summarise content better than we can
- this is a small part of a bit of cleanup of our test files
- the goal is to make the existing tests clearer with a view to making it easier to write more tests
- this makes the test structure follow the codebase structure more closely
- eventually we will colocate the frontend tests with the frontend code
