Mention canary flag as pre-requisite for MRVA

Author: elenatanasoiu

extensions/ql-vscode/docs/test-plan.md

@@ -16,6 +16,10 @@ choose to go through some of the Optional Test Cases.
 
 ## Required Test Cases
 
+### Pre-requisites
+
+- Flip the `codeQL.canary` flag. This will enable MRVA in the extension.
+
 ### Test Case 1: Running a problem path query and viewing results
 
 1. Open the [UnsafeJQueryPlugin query](https://github.com/github/codeql/blob/main/javascript/ql/src/Security/CWE-079/UnsafeJQueryPlugin.ql).