Fix: Test 560 did not Properly Use $CVMFS_TEST_REPO

**Note:** Based on [Feature: Garbage Collection](https://github.com/cvmfs/cvmfs/pull/605),  [Feature: Server Integration of the Garbage Collection](https://github.com/cvmfs/cvmfs/pull/606), [Feature: Add a MockHistory in the Unit Test Suite](https://github.com/cvmfs/cvmfs/pull/607), [Fix: Garbage Collection Code Base](https://github.com/cvmfs/cvmfs/pull/608) and [Feature: CatalogTraversal can have a Timestamp Threshold](https://github.com/cvmfs/cvmfs/pull/610).