test_json.py: Validate JSON files against JSON Schema

[jpmckinney]

2019 update: Running the following on each of the standard, profile, extension repos:

find . -name '*.json' -not -name 'extension.json' -not -name '*-schema.json' -not -path '*metaschema*' -not -path '*/_static/docson/*'

Only these have OCDS JSON files:

• standard/docs/en/examples
• standard/docs/en/examples/merging
• infrastructure/docs/examples
• public-private-partnerships/docs/examples (from OCDS Show)
• ocds_budget_and_spend_extension/examples
• ocds_contract_completion_extension/examples

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Off the top of my head, we'd need to:

• Determine whether this JSON file has an appropriate JSON Schema to validate against
• Determine which JSON Schema to use (based on the presence of keys and the version)
• Support extensions and automatically extend the JSON Schema

[Bjwebb]

Note that the python jsonschema library doesn't check formats by default, you need to set it up explicitly https://python-jsonschema.readthedocs.io/en/latest/validate/#validating-formats

[jpmckinney]

Can re-use / replace this test once done: https://github.com/open-contracting/sample-data/blob/master/tests/test_fictional_example.py