E2E Tests: Intermittent failure in sidebar resizing test

[aduth]

Previously: #20982 (comment)

Examples:

• https://travis-ci.com/github/WordPress/gutenberg/jobs/304777218
• https://travis-ci.com/github/WordPress/gutenberg/jobs/304592721
• https://travis-ci.com/github/WordPress/gutenberg/jobs/304537367
• https://travis-ci.com/github/WordPress/gutenberg/jobs/304482103
• https://travis-ci.com/github/WordPress/gutenberg/jobs/304477922

Failure message:

FAIL packages/e2e-tests/specs/editor/various/sidebar.test.js (16.039s)
  Sidebar
  ● Sidebar › should reopen sidebar the sidebar when resizing from mobile to desktop if the sidebar was closed automatically
    expect(received).toHaveLength(expected)
    Expected length: 1
    Received length: 0
    Received array:  []
      89 | 
      90 | 		const sidebarsDesktop = await page.$$( SIDEBAR_SELECTOR );
    > 91 | 		expect( sidebarsDesktop ).toHaveLength( 1 );
         | 		                          ^
      92 | 	} );
      93 | 
      94 | 	it( 'should preserve tab order while changing active tab', async () => {
      at Object.toHaveLength (specs/editor/various/sidebar.test.js:91:29)
          at runMicrotasks (<anonymous>)

Steps to Reproduce:

It is fairly reliably reproduced when using CPU throttling in end-to-end tests, and not without, indicating some issue of CPU timing.

THROTTLE_CPU=4 npm run test-e2e packages/e2e-tests/specs/editor/various/sidebar.test.js

Related history:

• Every commit on master since Apply the editor styles to the Site Editor page #20982 have failed; all but one of which fail at this specific test case. It was not previously known to be a common failure.