Build: Upgrade grunt-contrib-qunit and puppeteer

* test/overload.html

  Fix reliance on unsupported late-defined tests, instead separate
  capture and test, with test defined early.

  Fix reliance on conditionally executed tests. Define test early
  and unconditonally instead.

  Fix noise in CI output from supposed uncaught error,
  by returning true instead of false.

  Document reason for non-strict test, and make double load
  clearly intentional.

* sandboxed-iframe.html

  grunt-contrib-qunit v6.1.0 and later report uncaught errors,
  and the parent frame was failing since the injected 'bridge' script
  would fail to find QUnit.

  grunt-contrib-qunit v7.0.1 changed bridge.js to be a no-op in
  iframes and other windows, which means re-use of this internal
  script no longer works. It was never supported anyway.

  Instead, let the parent/child iframe communicate via postMessage,
  and have the parent window run as a normal test from there as far
  as CI is concerned.

Ref https://github.com/gruntjs/grunt-contrib-qunit.