Test invalid submissions are safely discarded

We just hit the same problem again with submissions: invalid I/O messages block the whole submission queue, because the `kcidb_load_queue` function throws an exception trying to load invalid data into the database. This shouldn't happen, and is being fixed, but we need a test that will prevent this from happening *again*.

The test should submit a message failing validation to the submission queue, followed (after a delay) by valid messages. The invalid message shouldn't make it, but the valid messages should.

Start by locating kcidb_load_queue and the submission-queue test entry point, then trace how validation failures are handled. Add a regression test that queues an invalid message, waits, and submits valid messages; done means the invalid message is discarded without blocking the later valid messages.

Written by the indexing model from the issue text.