Test data - Core

As discussed today:
- It would be helpful to have test data which can be used to verify correct implementation of CORE rules as well as structure and ingestion. There was also an occasional request for that.
- There are already some examples in documents section in Github. These include in some cases issues that are not (yet) conformant to the CORE rules.
- It is better to be uniform and have all test data in the same location.
- There is also test data in tabular format in CORE which is only useful as part of the core engine process (after transformation) and not as input.
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Up to the team to decide
- What and how much test data we like to make available
- Constraints?
- Where and how do we like to make it available.