Improve unit and integration test coverage

[zaneselvans]

Now that we've got CI running #46 we can start improving our test coverage! Some tests that we know we'd like to run:

• Extract Taxonomy metadata from zip archives stored in the repository
• Extract datapackage.json metadata from taxonomies stored in zip archives stored in the repository
• Extract SQLite DB from zip archives stored in the repository
• Test that LC_ALL environment variable is not altered by Arelle (this should be fixed but... just in case of regressions)
• Add test data & test cases for all the different FERC forms we are trying to support (this will allow us to skip these time consuming extractions in our fast integration tests in the main PUDL repo):
  • Form 1
  • Form 2
  • Form 6
  • Form 60
  • Form 714