Add views source context test coverage (styles, rows, unsaved views)

Problem/Motivation

Some behaviors of the views source plugins are not
pinned by any test:

- view_rows is only offered in styles, view_field only in rows.
- When a view is edited but not saved, source plugins read the unsaved view.
- hide_empty is read from the style or row plugin options. Every fixture has
hide_empty disabled and the tested field in first position, so a change of
where those options come from would not be caught.

Let's add some more tests :

- kernel tests for ui_patterns_views:
- sources availability depending on the context and views plugin type
- a saved view with a modified copy in the views shared tempstore: view_title returns the unsaved title.

- new functional tests:
- a view with two fields where the tested field is not in first position:
hide_empty from the plugin options, hide_empty from the field itself, an
excluded field..etc

- one end-to-end test: a field added in the views UI and not saved is offered
in the component form and rendered in the preview.