Fix appending text to Text node's data in parse5 adaptor

The "insert a character" algorithm [1] in the HTML parser is clear on
appending directly to the Text node's data rather than using the replace
data algorithm (which creates mutation records).

[1]: https://html.spec.whatwg.org/multipage/parsing.html#insert-a-character

Fixes: jsdom#3261

lib/jsdom/browser/parser/html.js

@@ -139,7 +139,7 @@ class JSDOMParse5Adapter {
   insertText(parentNode, text) {
     const { lastChild } = parentNode;
     if (lastChild && lastChild.nodeType === nodeTypes.TEXT_NODE) {
-      lastChild.data += text;
+      lastChild._data += text;
     } else {
       const ownerDocument = this._ownerDocument();
       const textNode = Text.createImpl(this._globalObject, [], { data: text, ownerDocument });
@@ -150,7 +150,7 @@ class JSDOMParse5Adapter {
   insertTextBefore(parentNode, text, referenceNode) {
     const { previousSibling } = referenceNode;
     if (previousSibling && previousSibling.nodeType === nodeTypes.TEXT_NODE) {
-      previousSibling.data += text;
+      previousSibling._data += text;
     } else {
       const ownerDocument = this._ownerDocument();
       const textNode = Text.createImpl(this._globalObject, [], { data: text, ownerDocument });