Address a few small conf_validations issues

[riley-harper]

This PR fixes a few issues in conf_validations:

• Added an f to a string that expected to be formatted but wasn't being formatted.
• Added a check to analyze_conf() to make sure that both data sources contain the id_column.
• Refactored some testing structure to allow for testing conf_validations more easily. This involved moving some files from the hlink_config directory, which I've now deleted, to hlink/tests/conf and creating a new hlink/tests/input_data/conf_validation directory. The config files used for testing conf_validations reference their datasource files in hlink/tests/input_data/conf_validation by relative paths from the root project directory.

[jacwellington]

This looks great!