Check for existance of elements under test in application_spec.js

## What does this MR do?

Checks that the elements under test in `application_spec.js` actually exist.

## Why was this MR needed?

```
$ echo 'just garbage' > spec/javascripts/fixtures/application.html.haml 
$ bundle exec teaspoon spec/javascripts/application_spec.js 
2 examples, 0 failures
```

See merge request !6051