Modify test so it is OK if class already loaded (rootmap)

A FWCore/FWLite unit test fails because, in a test of the loader, the test fails if the class is already loaded before the loader is invoked.  Since the introduction of rootmap files in CMS, it is expected that the class will already be loaded.  So this pull request simply modifies the test to return success in this case.
This test still has utility in ROOT 6 because it will still fail if the class is not loaded, so it serves as a test of the functionality of rootmap files.
Please merge this pull request as soon as convenient.