Turn off axe iframe example reporting

This matches axe behaviour from previous releases, but see discussion: #2442 (comment)

__tests__/accessiblity-audit.test.js

@@ -1,4 +1,3 @@
-
 const { AxePuppeteer } = require('@axe-core/puppeteer')
 
 const { setupPage } = require('../lib/jest-utilities.js')
@@ -24,6 +23,10 @@ async function analyze (path) {
     .exclude('.govuk-skip-link')
     // axe reports that the back to top button is not inside a landmark, which is intentional.
     .exclude('.app-back-to-top')
+    // axe reports that the frame "does not have a main landmark" and example <h1> headings
+    // violate "Heading levels should only increase by one", which is intentional.
+    // https://github.com/alphagov/govuk-design-system/pull/2442#issuecomment-1326600528
+    .exclude('.app-example__frame')
 
   return axe.analyze()
 }