Update test database dump should be based on beta 12 and contain content

Problem/Motivation

Opening this as critical because:
- it's blocking other criticals
- every critical issue that deals with it has commit conflicts with each other
- false positives in the upgrade path tests.

The current database dump for updates was taken from pre-beta 12, and now has some modifications to make it suitable for tests that have been added.

We have false-passes in update tests because the database dump has no nodes/taxonomy terms etc.

Additionally issues like #2540416: Move update.php back to a front controller are getting blocked on inconsistent data in the database dump.

Proposed resolution

Since we know that the update path from beta 12 either works, or doesn't work in a way that is documented in existing critical issues, we should target that for the database dump, and ensure there's some data to actually get upgraded - at least nodes, users, taxonomy terms, comments, forums, contact forms, custom block, blocks with visibility conditions.

Remaining tasks

1. Install beta 12
2. Install every core module
3. Enable some language
4. Create content - at least the following:
1. nodes (including a file and/or image upload?)
2. comments
3. users
4. taxonomy terms
5. forums
6. contact forms
7. custom blocks
8. blocks with visibility conditions
9. content translations for at least some of the above items
10. anything else you can think of
5. Translate some configuration.
6. Translate some strings via locale.
7. Dump the database:
1. core/scripts/dump-database-d8-mysql.php | gzip > core/modules/system/tests/fixtures/update/drupal-8.bare.standard.php.gz
8. Create a patch.

User interface changes

API changes

Data model changes