Add e2e tests to cover onSplit behaviour for headings (WordPress#10326)

This ensures that if a user types a heading, then moves the caret to the beginning of the line and presses ENTER, that a paragraph block is created above. Also ensures that when a user presses ENTER at the end of a heading, a paragraph block is created below.

packages/e2e-tests/specs/__snapshots__/splitting-merging.test.js.snap

@@ -1,5 +1,25 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`splitting and merging blocks should create a paragraph block above when pressing enter at start of heading 1`] = `
+"<!-- wp:paragraph -->
+<p></p>
+<!-- /wp:paragraph -->
+
+<!-- wp:heading -->
+<h2>Abc</h2>
+<!-- /wp:heading -->"
+`;
+
+exports[`splitting and merging blocks should create a paragraph block below when pressing enter at end of heading 1`] = `
+"<!-- wp:heading -->
+<h2>Abc</h2>
+<!-- /wp:heading -->
+
+<!-- wp:paragraph -->
+<p></p>
+<!-- /wp:paragraph -->"
+`;
+
 exports[`splitting and merging blocks should delete an empty first line 1`] = `
 "<!-- wp:paragraph -->
 <p>First</p>

packages/e2e-tests/specs/splitting-merging.test.js

@@ -210,4 +210,24 @@ describe( 'splitting and merging blocks', () => {
 
 		expect( isInDefaultBlock ).toBe( true );
 	} );
+
+	it( 'should create a paragraph block above when pressing enter at start of heading', async () => {
+		await insertBlock( 'Heading' );
+		await page.keyboard.type( 'Abc' );
+		await pressKeyTimes( 'ArrowLeft', 3 );
+		await page.keyboard.press( 'Enter' );
+		await page.keyboard.press( 'ArrowUp' );
+
+		const activeElementClassList = await page.evaluate( () => document.activeElement.classList );
+		expect( Object.values( activeElementClassList ) ).toContain( 'wp-block-paragraph' );
+		expect( await getEditedPostContent() ).toMatchSnapshot();
+	} );
+
+	it( 'should create a paragraph block below when pressing enter at end of heading', async () => {
+		await insertBlock( 'Heading' );
+		await page.keyboard.type( 'Abc' );
+		await page.keyboard.press( 'Enter' );
+
+		expect( await getEditedPostContent() ).toMatchSnapshot();
+	} );
 } );