Problem/Motivation
The D7 test fixture has been steadily getting more difficult too work with, my guess is that this started when the translation work began. It is no longer possible to bring up a site with the fixture and do things through the UI. Attempts to save or create anything or clear cache result in 'An illegal choice has been detected. Please contact the site administrator.'
To reproduce:
1. Use db-tools to import the fixture to D7 site
2. navigate to /admin/config/development/performance
3. Click 'Clear cache'
4. Error msg, "An illegal choice has been detected. Please contact the site administrator."
Since this task blocks all, or nearly all, D7 migration work, marking as Major and a blocker.
Proposed resolution
Fix the D7 test fixture so it is usable via the UI.
Updating the fixture to the latest D7 or adding data is to be done in #3022137: Update migrate fixtures for remaining active issues.
Remaining tasks
Do it.