[Patch v2 5/6] test: add generator for random "stub" messages

Initial use case is testing dump and restore, so we only have message-ids and tags. The message ID's are nothing like RFC compliant, but it doesn't seem any harder to roundtrip random UTF-8 strings than RFC-compliant ones. Tags are UTF-8, even though notmuch is in principle more generous than tha

[Patch v2 5/6] test: add generator for random "stub" messages

Initial use case is testing dump and restore, so we only have message-ids and tags. The message ID's are nothing like RFC compliant, but it doesn't seem any harder to roundtrip random UTF-8 strings than RFC-compliant ones. Tags are UTF-8, even though notmuch is in principle more generous than tha