Changed return types, identifiers and added pointers to parents

Several changes have been made:
 * Added some missing identifiers, and specified their type in a
   consistent way
 * TestItem returns ReturnCode, TestCase finalizes a Report
   containing a list of ReturnCodes for all TestItems run
 * TestCase has a pointer to parent TestPlan, that in turn has a
   pointer to parent TestBenchConfiguration
 * Fixed some null pointer checks
 * Changed tests accordingly

Signed-off-by: Raffaele Aquilone <aquiloneraf@gmail.com>