E2E Tests: Improve Allowed Inner Blocks test stability

[aduth]

Fixes #21052

This pull request seeks to improve the stability of the "Allowed Blocks Setting on InnerBlocks" end-to-end test which has proven to be a source of common intermittent failures.

Implementation Notes:

As mentioned at #21052 (comment), the suspected issue is that the test relies on an update of its child blocks to apply new settings to its own InnerBlocks rendering. As of #19343, the parent's rendering occurs asynchronously and may be delayed.

The proposed changes seek to resolve this by adding a new data- attribute to the parent block's rendering, which is used by the test to know the point at which the next render is completed.

Testing Instructions:

Ensure end-to-end tests pass:

npm run test-e2e packages/e2e-tests/specs/editor/plugins/inner-blocks-allowed-blocks.test.js

[github-actions]

Size Change: 0 B

Total Size: 856 kB

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.43 kB	0 B
build/api-fetch/index.js	3.39 kB	0 B
build/autop/index.js	2.58 kB	0 B
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build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	760 B	0 B
build/block-editor/index.js	101 kB	0 B
build/block-editor/style-rtl.css	10.9 kB	0 B
build/block-editor/style.css	10.9 kB	0 B
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build/block-library/index.js	110 kB	0 B
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build/block-library/style.css	7.44 kB	0 B
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build/block-library/theme.css	671 B	0 B
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build/components/style-rtl.css	15.8 kB	0 B
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build/compose/index.js	6.21 kB	0 B
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build/data/index.js	8.25 kB	0 B
build/date/index.js	5.37 kB	0 B
build/deprecated/index.js	771 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.06 kB	0 B
build/edit-post/index.js	91.2 kB	0 B
build/edit-post/style-rtl.css	8.43 kB	0 B
build/edit-post/style.css	8.43 kB	0 B
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build/edit-site/style.css	2.9 kB	0 B
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build/editor/style.css	3.38 kB	0 B
build/element/index.js	4.44 kB	0 B
build/escape-html/index.js	733 B	0 B
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build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	1.93 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.49 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
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build/keycodes/index.js	1.69 kB	0 B
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build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	4.84 kB	0 B
build/notices/index.js	1.57 kB	0 B
build/nux/index.js	3.01 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.54 kB	0 B
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build/url/index.js	4.01 kB	0 B
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build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.18 kB	0 B

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