test: add conditional compilation flag for datanode mock module (#2130)

src/datanode/src/lib.rs

@@ -21,7 +21,7 @@ mod greptimedb_telemetry;
 pub mod heartbeat;
 pub mod instance;
 pub mod metrics;
-#[cfg(feature = "testing")]
+#[cfg(any(test, feature = "testing"))]
 mod mock;
 pub mod server;
 pub mod sql;