Improve tests for "schema_builder" and "_file_config"

[stephen-bunn]

We should have better test coverage for schema_builder.py and _file_config.py as those files contain 90% of our logic. Currently we do basic testing for building basic configs and their schemas. We don't deal with edge cases for unnormal config instances.

Tests should focus on:

• Testing all config to schema types when building schemas
• Testing schema building for nested configs
• Testing / validating the generated jsonschema metadata properites