Update 'Data-driven tests' design document

[mradbourne]

This PR updates the testing design document to capture the decisions and conversation around testing since November.

The main updates are:

• Update suggestions around test format to explain the rationale for JSON.
• Describing the test schema.
• Referencing test functions.
• Removing example tests from the document because we have test files in the repo that illustrate usage.

[eemeli]

Looks good; see inline comments.