Updated _Sidebar (markdown)

_Sidebar.md

@@ -26,7 +26,9 @@
 * [[Feedback Channels]]
 * [[Source Code Organization]]
 * [[Coding Guidelines]]
-* [[Smoke Test]]
+* Testing
+  * [[Writing Tests]]
+  * [[Smoke Test]]
 * [[Dealing with Test Flakiness]]
 * [[Contributor License Agreement]]
 * [[Extension API Guidelines]]