feat(cdk/testing): add option to customise TestElement#text

[bgotink]

Feature Description

We use web components in our angular components. We can override the browser's querySelectorAll in the testbed test environment to pierce through shadow roots. We would like to also be able to override the innerText/textContent functionality (for TestElement#text) of the browser as the builtin innerText and textContent properties skips over shadow content:

<angular-component>
  Hello,
  <web-component>
    <template shadowrootmode="open">World!</template>
  </web-component>
</angular-component>

--> document.querySelector('angular-component').innerText === 'Hello, '

Use Case

Improve unit testing using component harnesses when web components using shadow DOM are used.