Article commercial tests

See also #2621

@kenlim 

As discussed, here's two simple tests that alternate the position of the commercial components and other adverts.

In each test there's two variant that creates ad-slots on the page with [alternating names](https://github.com/guardian/frontend/pull/2625/files#diff-392eec1cfb0a4964c846428411826dddR51), ready for the ad server to populate.

The two tests,

 - commercial-in-article-mobile - scoped to _mobile_ breakpoint.
 - commercial-in-article-desktop - scoped to _desktop_ and _wide_ breakpoints.

The only thing remaining is to do is the commercial-in-article-desktop top slot ... 

https://github.com/guardian/frontend/pull/2625/files#diff-ac92748c1979c9acd855a88d3e5e31e7R17

... which I wasn't sure what to do with.

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Also, OAS seems to be sending back the correct slots - see the Middle1 slot contains my commercial component.

http://oas.theguardian.com/RealMedia/ads/adstream_mjx.ads/www.theguardian-alpha1.com/travel/2013/nov/26/working-farm-holidays-around-the-world/oas.html/376877533@Top,Middle,Middle1,Bottom?k=working-holidays&k=travel&k=gap-year-travel&k=budget-travel&pt=article&ct=article&cat=travel&a=E013053