Jetpack tests require site on free plan

[alisterscott]

The Jetpack tests require a site on a free plan, so that it specs can compare plans etc.

But this has some downsides like Akismet isn't active so the sites get a lot of unfiltered spam.

This is to investigate how we have have sites on Jetpack plans and still cover the plans functionality etc.

[hoverduck]

I think we'll need to have a parameter in NODE_CONFIG that details how the current site-under-test is set up to dictate which tests are run. In #643 I have an update to default.json that says whether the site is https or not, and this would fit nicely with that.

[alisterscott]

The payment button test introduced in #666 is not currently run on Jetpack sites as this requires a paid Jetpack plan - so when this issue is resolved we should add @jetpack to it