Need script tests

The current test harness only tests the shared part of the archiver which parses emails.

There need to be some tests for the functioning of the archiver when processing input messages, and for the importer when uploading mail from files.

This will either need a test ES instance, or a way of emulating the behaviour of the ElasticSearch API. Or both.

Start with the shared test harness and trace how the archiver processes input messages and how the importer uploads mail from files. Determine whether a test Elasticsearch instance, an Elasticsearch API emulator, or both are needed. Done means coverage exists for both script paths, including their external search behavior.

Written by the indexing model from the issue text.