When adding test data, hack GroupCount timestamp

Because the MessageTracker in briar initializes the GroupCount timestamp
to the current time when the contact is being added, we cannot have
realistic test data in the database where the GroupCount timestamp
matches that of the last message in the chat history. This is a dirty
workaround to make this possible.
1 job for !54 with 103-hack-group-timestamps in 1 minute and 39 seconds (queued for 8 seconds)
latest detached
Status Job ID Name Coverage
  Test
passed test #13267

00:01:39