CRM-12321 - SqlGroupTest - Test prefetching more directly.

When multi-tier caching was introduced, testPrefetch regressed because it
relied on side-effects from having totally independent caches -- but the
multi-tier cache added extra, unforeseen sharing between SqlGroup instances.
In reality, prefetching works. This revision tests prefetch more directly
to workaround the changed assumptions.

----------------------------------------
* CRM-12321: Multi-tier caching causes multiple test regressions
  http://issues.civicrm.org/jira/browse/CRM-12321