HTML Reporter: Change #qunit-banner from H2 to DIV

There is no need for this presentational pixel bar to be a heading
element. It is completely a presentational thing that is better
represented with a DIV instead.

This previously produced the following accessiblity warning,
e.g. in Firefox's Accessibility inspector, or in Chrome with AXE.

> Headings must not be labeled.
>
> `Role: heading. "" [text label]`
>
> https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names

> Headings must not be empty.
>
> https://github.com/dequelabs/axe-core/blob/v4.9.1/doc/rule-descriptions.md
> https://act-rules.github.io/rules/ffd0e9

Fixes #1427.

docs/browser.md

@@ -203,7 +203,7 @@ QUnit 1.x, 2.x:
 ```html
 <div id="qunit">
   <h1 id="qunit-header">…</h1>
-  <h2 id="qunit-banner"></h2>
+  <div id="qunit-banner"></div>
   <div id="qunit-testrunner-toolbar">…</div>
   <h2 id="qunit-userAgent">…</h2>
   <div id="qunit-testresult">…</div>

src/reporters/HtmlReporter.js

@@ -666,7 +666,7 @@ export default class HtmlReporter {
       // contains id="qunit".
       qunit.innerHTML =
       "<h1 id='qunit-header'>" + escapeText(document.title) + '</h1>' +
-      "<h2 id='qunit-banner'></h2>" +
+      "<div id='qunit-banner'></div>" +
       "<div id='qunit-testrunner-toolbar' role='navigation'></div>" +
       this.appendFilteredTest() +
       "<h2 id='qunit-userAgent'></h2>" +