Cleaning on tests

• Try to remove test fixtures and add migrations
• Replace faker for longitude/latitude by proper faker
• Rename fixtures depending on datacite raw or form raw.
• Factorize and simplify tests for easier comprehension:
  • metadata_serializer should not test sub-object data. only labels, length, and type.
  • sub-object_serializer should test sub-object data
• Split tests in unitary and integration tests.