Add buffer integration test

Rearrange channel crate structure
Get channel buffer from database

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

crates/collab/src/db.rs

@@ -1,7 +1,8 @@
 #[cfg(test)]
-mod db_tests;
+pub mod tests;
+
 #[cfg(test)]
-pub mod test_db;
+pub use tests::TestDb;
 
 mod ids;
 mod queries;