Change type of block used in e2e test to a separator

As mentioned in the comment above, this test will not work for blocks that
contain an input. The image block now contains a url input field causing
the test to fail.

Co-authored-by: imath <math.viet@gmail.com>

Author: talldan

test/e2e/specs/__snapshots__/adding-blocks.test.js.snap

@@ -9,9 +9,9 @@ exports[`adding blocks Should insert content using the placeholder and the regul
 <blockquote class=\\"wp-block-quote\\"><p>Quote block</p></blockquote>
 <!-- /wp:quote -->
 
-<!-- wp:image -->
-<figure class=\\"wp-block-image\\"><img alt=\\"\\"/></figure>
-<!-- /wp:image -->
+<!-- wp:separator -->
+<hr class=\\"wp-block-separator\\"/>
+<!-- /wp:separator -->
 
 <!-- wp:paragraph -->
 <p></p>

test/e2e/specs/adding-blocks.test.js

@@ -30,10 +30,10 @@ describe( 'adding blocks', () => {
 		await page.keyboard.press( 'ArrowDown' );
 
 		// Focus should be moved to block focus boundary on a block which does
-		// not have its own inputs (e.g. image). Proceeding to press enter will
+		// not have its own inputs (e.g. separator). Proceeding to press enter will
 		// append the default block. Pressing backspace on the focused block
 		// will remove it.
-		await page.keyboard.type( '/image' );
+		await page.keyboard.type( '/separator' );
 		await page.keyboard.press( 'Enter' );
 		await page.keyboard.press( 'Enter' );
 		expect( await getEditedPostContent() ).toMatchSnapshot();