Add buffer integration test

Rearrange channel crate structure
Get channel buffer from database

co-authored-by: Max <max@zed.dev>

crates/collab/src/db/queries.rs

@@ -9,6 +9,3 @@ pub mod rooms;
 pub mod servers;
 pub mod signups;
 pub mod users;
-
-#[cfg(test)]
-pub mod buffer_tests;