[Backport to release/11.1] e2e tests: Fix create-post e2e test against Gutenberg nightly

[woocommercebot]

This PR is a cherry-pick of #67803 to release/11.1.

Original PR Description

Changes proposed in this Pull Request:

The nightly Gutenberg e2e job has failed on create-post.spec.ts since 12 August. Gutenberg WordPress/gutenberg#81231 replaced the default block appender with a real empty paragraph block. The element keeps the accessible name Add default block, but its role changed from button to document. The test asked for the button role, so its locator matched nothing and timed out.

The test now matches by label:

await canvas.getByLabel( /Add default block|Empty block/ ).click();

create-page.spec.ts and products.block_theme.spec.ts already do this, after an earlier appender change in #53100. create-post.spec.ts was the last one still asking for the button role.

How to test the changes in this Pull Request:

1. Check out this branch.
2. Build the plugin: pnpm --filter=@woocommerce/plugin-woocommerce build.
3. Start the e2e environment: pnpm --filter=@woocommerce/plugin-woocommerce env:e2e.
4. Run against Gutenberg nightly: pnpm --filter=@woocommerce/plugin-woocommerce test:e2e:gb-nightly tests/wp-core/create-post.spec.ts. It passes. On trunk it fails at create-post.spec.ts:33 with TimeoutError: locator.click: Timeout 20000ms exceeded.
5. Run against Gutenberg stable: pnpm --filter=@woocommerce/plugin-woocommerce test:e2e:gb-stable tests/wp-core/create-post.spec.ts. It still passes.

Tested: https://github.com/woocommerce/woocommerce/actions/runs/32123269803

Milestone

• Automatically assign milestone for the next WooCommerce version

[github-actions]

Testing Guidelines

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