A bunch of fixes for a bunch of tests

[josh-feather]

In collaboration with @tbeadle.

• Fix file order precedence in Config load. Previously files in the CUCKOO_ROOT conf.d directories would be loaded after files in the CUSTOM_CONF_DIR directory. Let me know if this was intentional and I'll update the PR.
  • As part of this, introduce a env var to allow tests to skip loading of custom configs. This should, hopefully, make testing more consistent across the board.
• Flush the Error table on database test setup to prevent consecutive runs resulting in test failures.
• Make tests use the default config files unless otherwise required.
• Add additional skip criteria for file_extra_info tests. Some of the samples aren't in the tests/data submodule, so they failed if it was present locally.
• Move the Config() load out of file_extra_info_modules/__init__ into the file_extra_info_modules/__init__::extractor_ctx function so config isn't loaded prematurely. This gives us the opportunity to manipulate the config that is used in the tests.