[META] Improve test coverage in entity browser

Test coverage overview (always work in progress). Striked-through items are already done.

Basic functionality

• Basic CRUD operations on entity browser
• Validators
• Existing selection
• Default widget
• Dynamically generated permissions
• Dynamically generated routes

Configuration UI

Display plugins

• iFrame
• Modal
• Standalone

Selection display

• No display
• Multi step display (not in main ConfigUI class but in its own)
• View selection display (partly tested in EntityBrowserTest.php)

Widget

• View
• Upload
• Entity form

Individual plugins functionality

Widgets

• Upload (partly tested in PluginsTest.php)
• View (partly tested in PluginsTest.php)
• Entity form

Selection displays

• Multi step display (partly tested in MultiDisplayTest.php)
• View
• No display

Widget selectors

• Tabs (partly tested EntityBrowserTest.php)
• Dropdown (partly tested EntityBrowserTest.php)
• Single

Displays

• iFrame (partly tested in EntityBrowserUITest.php and PluginsTest.php)
• Modal
• Standalone

Field widgets

• Entity reference
• File field: #2801163: Missing preview image in widget
• Image field: #2801163: Missing preview image in widget

Integrations

Inline entity form

• IEF use existing (partly tested in InlineEntityIntegrationTest.php, test full flow: entities are selected, IEF table is automatically reloaded, correct entities appear in it)
• High prio: Entity browser on a field inside IEF
• High prio: Entity browser on a field inside nested IEF (2+ levels up)
• High prio: Entity browser on a field inside IEF, more than one IEF field on the entity (open EB in one, update, open in the other, update, have both open at the same time, update, make sure referenced entities are correct - they are not leaking between fields)

Paragraphs

• High prio: Entity browser on a paragraph field
• High prio: Entity browser on a nested paragraph field
• High prio: Multi value paragraph, more than one value using EB, make sure values are not leaking between them