This introduces ember-a11y-testing to acceptance tests via a helper
wrapper that allows us to globally ignore rules that we can address
separately. It also adds fixes for the aXe rules that were failing.
As the angle bracket invocation RFC says:
> There is no dedicated syntax for passing an "else" block
> directly. If needed, that can be passed using the named
> blocks syntax.
https://github.com/emberjs/rfcs/blob/master/text/0311-angle-bracket-invocation.md#block
Unfortunately, using a contextual component doesn’t help as
the yield inside that component will still result in content
rendering that would show when the source isn’t empty. So
we decided to change the interface so you have to check
whether the source is empty before using it, which aligns with
how list-table works.
