In this PR (https://github.com/consul/consul/pull/4683) a new syntax was introduced
in the component specs to check that the component was not rendering.
It seems interesting to add this syntax to the rest of the cases and thus unify the way
we check that a component is not rendering.
Checking the `style` attribute fails in JavaScript tests because the
browser converts colors to the `rgb()` format.
So we're testing the generated HTML in a component test while
simplifying the system test.
Now the banner component accepts either a banner or a section and loads
the banner if it's a section, so we don't have to add the `@banners`
variable in several controllers.
