Fix rule: ember/template-no-unused-block-params

[tcjr]

This PR updates the ember/template-no-unused-block-params to work correctly when the block param is used in attribute values.

For example, the following template is valid, but currently fails lint:

{{#each cats as |cat|}}
  <div class="cat {{cat.name}}"></div>
{{/each}}

With a scope of cats = [{ name: 'Stimpy' }], it produces this HTML:

<div class="cat Stimpy"></div>

Similarly, this template is valid:

{{#each cats as |cat|}}
  <div class="cat {{if (isFavorite cat.name favoriteCatNames) 'favorite'}}"></div>
{{/each}}

I've added 3 new valid template tests (2 of which fail without this fix), and 2 invalid template tests.