fix(moc-doc): make Node.contains() return true for self (#3150)

`Node.contains()` should return `true` if the passed node is the 
parent/context node.  [The wording in the spec](https://dom.spec.whatwg.org/#dom-node-contains) is that it
should return `true` if the passed node is an "inclusive descendant",
which differs from a regular "descendant" in that it includes the 
object itself (not just the child nodes).

src/mock-doc/node.ts

@@ -138,6 +138,9 @@ export class MockNode {
   }
 
   contains(otherNode: MockNode) {
+    if (otherNode === this) {
+      return true;
+    }
     return this.childNodes.includes(otherNode);
   }