[HYGIENE] Re-organise test fixtures

[madpah]

We're getting a large number of test fixtures now and and this ticket will re-organise them into sub-directories:

tests/fixtures/<FORMAT>/<SCHEMA_VERSION>/<FIXTURE_NAME>.<EXTENSION>

Where:

• <FORMAT> is json or xml
• <SCHEMA_VERSION> is decimalised schema version - e.g. 1.4
• <FIXTURE_NAME> remains a suitable descriptive name
• <EXTENSION> remains either json or xml