Pass the selector to this.$(...) helper in component tests

I tried doing `this.$('.some-child-element-in-the-component')` in my component tests and it actually ignored the selector and just returned the component, which was very confusing. I applied this patch on my setup and it seems to work fine.

However, when I tried to run the tests locally, I'm getting a lot of unrelated (I think?) failures/errors, so I'm unsure if I actually broke something.