Remove children of Document even if doc not in mirror (#923)

* Remove children of Document even if doc not in mirror

* fix flaky test

* Update packages/rrdom/test/diff.test.ts

Co-authored-by: Yun Feng <yun.feng@anu.edu.au>

Co-authored-by: Yun Feng <yun.feng@anu.edu.au>

packages/rrdom/src/diff.ts

@@ -316,7 +316,7 @@ function diffChildren(
          * We should delete it before insert a serialized one. Otherwise, an error 'Only one element on document allowed' will be thrown.
          */
         if (
-          replayer.mirror.getMeta(parentNode)?.type === RRNodeType.Document &&
+          parentNode.nodeName === '#document' &&
           replayer.mirror.getMeta(newNode)?.type === RRNodeType.Element &&
           (parentNode as Document).documentElement
         ) {