Log request count in cache helper tests
Log request count in cache helper tests

Include the request iteration in the error so that it's easier to identify
which request was incorrect. This matches what we've done in TestMisc and
TestFailover.

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