E2E Tests: Intermittent failure in Image block tests

[aduth]

Examples:

• https://travis-ci.com/github/WordPress/gutenberg/jobs/300232210
• https://travis-ci.com/github/WordPress/gutenberg/jobs/300231684
• https://travis-ci.com/github/WordPress/gutenberg/jobs/300200651
• https://travis-ci.com/github/WordPress/gutenberg/jobs/300137973

FAIL packages/e2e-tests/specs/editor/blocks/image.test.js (8.134s)
  Image
    ✓ can be inserted (3648ms)
    ✕ should replace, reset size, and keep selection (3905ms)
  ● Image › should replace, reset size, and keep selection
    expect(received).toMatch(expected)
    Expected pattern: /<!-- wp:image {"id":\d+,"sizeSlug":"large"} -->\s*<figure class="wp-block-image size-large"><img src="[^"]+\/e8de60ce-3010-4064-adaf-3c284ec05fbc\.png" alt="" class="wp-image-\d+"\/><\/figure>\s*<!-- \/wp:image -->/
    Received string:  "<!-- wp:image {\"id\":280,\"sizeSlug\":\"large\"} -->
    <figure class=\"wp-block-image size-large\"><img src=\"blob:http://localhost:8889/3282e96a-704d-41c6-a909-9442adfd8eba\" alt=\"\" class=\"wp-image-280\"/></figure>
    <!-- /wp:image -->"
      76 | 			`<!-- wp:image {"id":\\d+,"sizeSlug":"large"} -->\\s*<figure class="wp-block-image size-large"><img src="[^"]+\\/${ filename2 }\\.png" alt="" class="wp-image-\\d+"/></figure>\\s*<!-- \\/wp:image -->`
      77 | 		);
    > 78 | 		expect( await getEditedPostContent() ).toMatch( regex3 );
         | 		                                       ^
      79 | 
      80 | 		await page.keyboard.press( 'Backspace' );
      81 | 
      at Object.toMatch (specs/editor/blocks/image.test.js:78:42)
          at runMicrotasks (<anonymous>)

Recent changes (history):

• Add e2e test for image block #20622 (cc @ellatrix , @youknowriad)