Tests: Add a test to check that diagram JSON is valid

[duncandewhurst]

#212 adds diagrams which are generated from snippets of RDLS JSON data.

We should add a test to check that the JSON snippets are valid RDLS data and do not contain additional fields.

To do that, the test will need to preprocess the schema in order to:

• Make all fields optional
• Disallow additional properties everywhere
• Allow empty strings, objects and arrays

[duncandewhurst]

Following the changes in f3a9c30, we'll also need to relax type validation for the JSON snippets in order to allow empty strings in place of objects and arrays.