Share one render loop fixture across the render failure tests

Four test files built the same widget on CesiumWidget.prototype, viewer
proxy, store and notice; renderLoopFixture.mjs now does it once. The notice
test drives Cesium.Event instead of a hand-made double. frameEvents() drops
tileset.tileLoad, which Cesium raises inside its own error handling, and the
notice no longer guards a viewer without a canvas or requestRender.

scripts/format-scope.json

@@ -563,6 +563,7 @@
   "src/ui/renderFailureEventRetry.test.mjs",
   "src/app/renderFailureInlineRetry.test.mjs",
   "src/ui/renderFailureSecondLoop.test.mjs",
+  "src/ui/renderLoopFixture.mjs",
   "src/layers/cctv/lifecycle.js",
   "src/layers/cctv/controls.js",
   "src/layers/cctv/geometry.js",