Record test start in private API recorder

This way we can use the same JSON output to find out which tests don’t
use any private APIs.

test/common/modules/private_api_recorder.js

@@ -158,6 +158,10 @@ define([], function () {
           console.log(`(${loggingDescription}) Recorded private API: ${privateAPIIdentifier}`);
           this.privateAPIUsages.push({ context, privateAPIIdentifier });
         },
+        recordTestStart: () => {
+          console.log(`(${loggingDescription}) Recorded test start`);
+          this.privateAPIUsages.push({ context, privateAPIIdentifier: null });
+        }
       };
     }
 

test/common/modules/shared_helper.js

@@ -90,6 +90,10 @@ define([
       return new this(this.createContext(context));
     }
 
+    recordTestStart() {
+      this.privateApiContext.recordTestStart();
+    }
+
     recordPrivateApi(identifier) {
       this.privateApiContext.record(identifier);
     }

test/support/root_hooks.js

@@ -23,6 +23,7 @@ define(['shared_helper'], function (Helper) {
   });
   beforeEach(function () {
     this.helper = Helper.forTest(this);
+    this.helper.recordTestStart();
   });
   beforeEach(function () {
     this.helper.clearTransportPreference();