Tests: Fix failing block hierarchy navigation e2e test

[gziolo]

Description

Originally reported in #15241 (comment)

There is an unrelated issue with e2e test:

https://travis-ci.com/WordPress/gutenberg/jobs/196208309#L1161

FAIL specs/block-hierarchy-navigation.test.js (7.863s)
  ● Navigating the block hierarchy › should navigate block hierarchy using only the keyboard
    expect(received).toMatchSnapshot()
    Snapshot name: `Navigating the block hierarchy should navigate block hierarchy using only the keyboard 1`
    - Snapshot
    + Received
    @@ -9,9 +9,13 @@
      <div class="wp-block-column"></div>
      <!-- /wp:column -->
      
      <!-- wp:column -->
      <div class="wp-block-column"><!-- wp:paragraph -->
    - <p>Third column</p>
    + <p></p>
      <!-- /wp:paragraph --></div>
      <!-- /wp:column --></div>
    - <!-- /wp:columns -->"
    + <!-- /wp:columns -->
    + 
    + <!-- wp:image -->
    + <figure class="wp-block-image"><img alt=""/></figure>
    + <!-- /wp:image -->"
      80 | 		await page.keyboard.type( 'Third column' );
      81 | 
    > 82 | 		expect( await getEditedPostContent() ).toMatchSnapshot();
         | 		                                       ^
      83 | 	} );
      84 | 
      85 | 	it( 'should appear and function even without nested blocks', async () => {
      at Object.toMatchSnapshot (specs/block-hierarchy-navigation.test.js:82:42)
      at tryCatch (../../node_modules/regenerator-runtime/runtime.js:62:40)
      at Generator.invoke [as _invoke] (../../node_modules/regenerator-runtime/runtime.js:288:22)
      at Generator.prototype.(anonymous function) [as next] (../../node_modules/regenerator-runtime/runtime.js:114:21)
      at asyncGeneratorStep (specs/block-hierarchy-navigation.test.js:9:103)
      at _next (specs/block-hierarchy-navigation.test.js:11:194)

It looks like it's an easy fix. Missing await seems to be the root cause.

How has this been tested?

With the local Docker setup up and running (./bin/setup-local-env.sh):

for i in {1..20}; do npx wp-scripts test-e2e --config packages/e2e-tests/jest.config.js packages/e2e-tests/specs/block-hierarchy-navigation.test.js; done;

[aduth]

Related: #14219 (Seems I'd missed this one)