DOM: Add iframe insertion & removal steps WPTs

[chromium-wpt-export-bot]

To help resolve whatwg/dom#808, we need WPTs
asserting exactly when (DOM-observing) script can and cannot be invoked
during the insertion and removing steps for iframes and script elements.

The tests in this CL assert the current spec behavior of:

Iframe:

• Insertion:
  • Synchronously fire the load event in the iframe document
• Removal:
  • No script is run in between multiple iframe removals. Script
    cannot observe the state of the DOM in between multiple
    synchronous removals because, i.e., no unload events are fired
    in this case per HTML 1.
    Script:
• Insertion:
  • Synchronously execute <script> elements upon insertion, even in
    between the insertions of individual <script> elements that are
    added "atomically" by a single DocumentFragment insertion. Note
    that Chromium and Gecko fail this test. In the DocumentFragment
    case, both of these browsers insert all <scripts> into the DOM
    before executing any of them. This means that once the scripts
    start executing, they can all observe each other's participation
    in the DOM tree.

At the moment, there is ongoing discussion 2 about the possibility of
changing the DOM/HTML Standard's model to more-closely match what Gecko
and Chromium do with "atomic" DOM insertion (i.e., running script as a
side effect very explicitly after all DOM node insertion is done).

R=[email protected]

Bug: 40150299
Change-Id: Iff959bbb0d32d772ae7162d5d9e54a5817959086
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5251828
Reviewed-by: Mason Freed <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1264406}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.