fix(sandbox): add `allow-forms` so <dialog /> element could work properly

[hanyujie2002]

Summary

Fixes #11692 Fixes #11460

Problem

<dialog /> element doesn't work as expected in example of https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dialog#html-only_dialog. With chrome console, Blocked form submission to '' because the form's frame is sandboxed and the 'allow-forms' permission is not set. is reported.

Solution

I add allow-forms to the sandbox in embed live example file.

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Before

After

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[hanyujie2002]

P.S. Five months ago I met and fixed a similar problem in playground https://github.com/mdn/yari/issues/10788

[hanyujie2002]

I am not sure if I should update the test code as well

[fiji-flo]

fixed in https://github.com/mdn/rari/commit/dd5ac867f5ffc55793ccc0837ec6f11282c6fa78

Thank you 🙏