Add tests and fix behavior of scanner cli

• add smoke and unit tests for cli to have maximum coverage
• fix behavior of cli in case an invalid config file is specified via envvar
• rename scan to do_scan in scanner
• fix not self-contained temp_path fixture in conftest

Need advice on whether to use a real (tree that exist in the archive) root_path for the tests, here do_scan is mocked and never actually called.

Follow-up to !31 (closed).

Related to !31 (closed), D4130

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Migrated from D4089 (view on Phabricator)