Move HTTP client to var
Move HTTP client to var

So that we can re-use it across tests. Meaning that we don't have to
redefine it, and it's more efficient.

mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests