test: user istanbul ignore next due to nyc issue

lib/core/middleware/error.js

@@ -102,7 +102,8 @@ async function readSource(frame) {
   }
 
   // Fallback: use server bundle
-  /* istanbul ignore if */
+  // TODO: restore to if after https://github.com/istanbuljs/nyc/issues/595 fixed
+  /* istanbul ignore next */
   if (!frame.contents) {
     frame.contents = this.resources.serverBundle.files[frame.fileName]
   }