Migrate separator block tests to Playwright (#41130)

* Migrate separator block tests to Playwright

* Remove use of clickBlockAppender() utils

* Use locators where possible

* Use page.click()

Co-authored-by: Kai Hao <kevin830726@gmail.com>

test/e2e/specs/editor/blocks/__snapshots__/Separator-can-be-created-by-three-dashes-and-enter-1-chromium.txt

@@ -0,0 +1,3 @@
+<!-- wp:separator -->
+<hr class="wp-block-separator has-alpha-channel-opacity"/>
+<!-- /wp:separator -->

test/e2e/specs/editor/blocks/separator.spec.js

@@ -0,0 +1,21 @@
+/**
+ * WordPress dependencies
+ */
+const { test, expect } = require( '@wordpress/e2e-test-utils-playwright' );
+
+test.describe( 'Separator', () => {
+	test.beforeEach( async ( { admin } ) => {
+		await admin.createNewPost();
+	} );
+
+	test( 'can be created by three dashes and enter', async ( {
+		editor,
+		page,
+	} ) => {
+		await page.click( 'role=button[name="Add default block"i]' );
+		await page.keyboard.type( '---' );
+		await page.keyboard.press( 'Enter' );
+
+		expect( await editor.getEditedPostContent() ).toMatchSnapshot();
+	} );
+} );