createCanvasContextAsync: 在生命周期 onLoad 中获取，配合自动化测试

(cherry picked from commit 32b846c4)

pages/component/canvas/canvas.uvue

@@ -76,7 +76,7 @@
         testFrameCount: 0
       }
     },
-    onReady() {
+    onLoad() {
       // HBuilderX 4.25+
       // 异步调用方式, 跨平台写法
       uni.createCanvasContextAsync({
@@ -100,7 +100,8 @@
           this.testCanvasContext = true
         }
       })
-
+    },
+    onReady() {
       // 同步调用方式，仅支持 app/web
       // let canvas = uni.getElementById("canvas") as UniCanvasElement
       // this.renderingContext = canvas.getContext("2d")