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
that.

updated for id:m2wr04ocro.fsf@guru.guru-group.fi

- talk about Unicode value rather some specific encoding
- call talloc_realloc less times