Add documentation how to use safe components in tests

[mydea]

This PR adds a section to the safe component helper usage docs on how to use it in tests. It took me a bit to figure it out, so I figured it might help others to have this in there!

[ef4]

Thanks, this looks good.

Just wanted to mention that this is an area that gets nicer as soon as we have a syntax for strict mode templates. For example, with GJS (which you can try already with https://github.com/ember-template-imports/ember-template-imports) your example becomes:

import Component from '@glimmer/component';

test('my test', async function(assert) {
  this.testComponent = <template>Test content: {{@message}}</template>;
  await render(hbs`
    <MyComponent @display={{this.testComponent}} />
  `);
});

We don't need to show that in the docs yet as it's nonstandard. But that's where the design is heading.