Add mdJson generator to appropriate writer tests

[stansmith907]

For the recently published FGDC writer I build a library of functions to generate mdJson test scripts. Besides saving code space this should pay out when there are changes to the mdJson schema. A single change to the mdJson test generator will automatically propagate through all tests.

The changes will also provide:

• messages issued from methods
• support for ERROR/WARNING/NOTICE level messages
• support for 'forceValid' flag
• message text loaded from YAML files for easy changes and multi-language support
• support for context added to message to identify source of issue

Writers to update:

• FGDC
• ISO 19115-2
• ISO 19110
• mdJson (only mdJson generator applies)