feat: handle the downgrade region instruction (#2855)

* feat: handle the downgrade region instruction

* test: add tests for RegionHeartbeatResponseHandler

* refactor: remove unused code

src/common/meta/src/heartbeat/handler.rs

@@ -37,7 +37,7 @@ pub struct HeartbeatResponseHandlerContext {
 /// HandleControl
 ///
 /// Controls process of handling heartbeat response.
-#[derive(PartialEq)]
+#[derive(Debug, PartialEq)]
 pub enum HandleControl {
     Continue,
     Done,

src/common/meta/src/heartbeat/mailbox.rs

@@ -30,8 +30,8 @@ pub struct MessageMeta {
     pub from: String,
 }
 
-#[cfg(test)]
 impl MessageMeta {
+    #[cfg(any(test, feature = "testing"))]
     pub fn new_test(id: u64, subject: &str, to: &str, from: &str) -> Self {
         MessageMeta {
             id,