Automated tests: Refactor existing and add missing coverage

Problem/Motivation

With the refactoring in 2.x we need a whole bunch of new tests. Maybe it's a good moment to re-think our testing approach to make all of them easier to maintain and cover the most use cases possible.

Proposed resolution

✔️ 1) Kernel: 
✔️  - All API methods
✔️ 2) Configuration form:
✔️  - Ensure UI elements are the ones we expect
✔️  - Ensure they do what is expected from them
✔️    -> The functionality itself can be tested programmatically, not through UI
✔️ 3) Integration test (implicitly tests entity_reference and link plugins)
✔️  - Basic CRUD in referenced nodes: 
✔️    * add target in field
✔️    * have 2 different targets in same field
✔️    * have duplicated targets in same field
✔️    * remove from field
✔️    * delete target
✔️    * delete host
✔️  - Deleting field storage should clean all usage tracked on this field
✔️ 4) Embedded content test:
✔️  - Should test logic specific to plugins:
✔️    * Entity Embed
✔️        - Single and multiple-entity tracking on the same field
✔️    * LinkIt
✔️        - Single and multiple-entity tracking on the same field
✔️        - References to non-existing entities don't register usage
✔️    * HtmlLink
✔️        - Single and multiple-entity tracking on the same field
✔️        - References to non-existing entities don't register usage
✔️        - Absolute and relative URLs
✔️        - Aliased URLs
✔️  - Test a plugin that tracks 2 field types (2952174)
✔️ 5) Config entities test:
✔️  - Should test logic specific to plugins:
✔️    * Entity Reference (when pointing to webforms)
✔️    * Block Field
✔️ 6) List controller test:
✔️  - All elements on the page are the ones we expect
✔️  - Correctly filter out garbage from `entity_usage`
✔️ 7) RevisionsTranslations test:
✔️  - Revisions are tracked correctly
✔️  - Translations are tracked correctly
✔️  - They work as expected in combination :) (2954638)
✔️  - Deleting node with hundreds of revisions correctly release all usage records
✔️  - We don't show duplicate usage on usage page (if translation not affected)
✔️ 8) Views test
✔️  - Test view integration with content entities with numeric IDs
✔️ 9) Paragraphs test
✔️  - Basic usage tracking of media entity inside paragraph on a node (2952448)
✔️  - Usage page shows parent's label
✔️  - Usage page skips orphan paragraph
✔️  - Nested paragraphs correctly store usage, and usage page shows the first non-paragraph parents label
✔️ 10) Batch update test
✔️  - Batch update page has specific permission
✔️  - Batch operation works
✔️  - Batch operation respects settings from config form

Remaining tasks

User interface changes

API changes

Data model changes