Count sent messages in integration tests

This branch adds some extra checks to BriarIntegrationTest's sync methods to ensure that the sender is sending the expected number of messages.

A new sync method allows edge cases such as duplicate messages to be specified. A couple of tests covering these edge cases are updated. Methods for sharing and unsharing messages to test out-of-order delivery are promoted from AutoDeleteIntegrationTest to BriarIntegrationTest.

These changes have low priority, so if this branch conflicts with work in progress on #1834 (closed) and #1835 (closed) I'll rebase it to be merged after that work's finished.