Dialog: Tests need to shed backcompat

[gabrielschulhof]

tests/integration/dialog-extension/index.html must have backcompat turned off, but then toolbars either do not instantiate or instantiate as footers, because they require the enhancer backcompat, which picks up custom initSelector values.

So, we must first land the toolbar review before we can remove the backcompat from these tests.

[gabrielschulhof]

We must turn off backcompat in the following (non-exhaustive list of) tests after we land toolbar:

• tests/integration/page/index.html
• tests/integration/dialog/index.html
• tests/integration/dialog-extension/index.html