Define criteria for tables/columns to be provided in example-data CSV files

[alarthast]

• See Slack conversion and this comment in Fix capitalisation in example data #2143.
• Currently it is unclear what tables should be included as CSV files in the example data. @evansd suggested that all core tables + all tables used in the tutorial would make sense
  • If so, we should add a CSV file representing the tpp.apcs table
• Once the criteria is set, the test file for example data should be updated to remove the hard-coding of tables.
• We should also look at whether our example CSV files ought to include all the columns of the tables they represent
  • If so, add the columns and use allow_missing_columns=False in the tests

[evansd]

We've decided to take the approach of:

1. expanding the core tables to include practice_registrations;
2. rewriting the more complex tutorial definition to only use core tables.

This allows us to make the policy very simple:

• The tutorial examples should only use core tables.
• We should provide example data for all core tables.