Review GTestSuites and GTestSuite interfaces.

GTestSuites
  The GTestSuites class now derives from GContainer and implements
  the usual container class methods (at(), size(), isempty(),
  set(), append(), insert(), remove(), reserve() and extend()).
  The operator[] now returns pointers to GTestSuite. The name()
  and timestamp() methods now return const references.

GTestSuite
  The GTestSuite class now derives from GBase. This implies that
  the clone() method needs to be implemented for derived classes.
  The test_assert() method now takes a const reference for the
  boolean argument. The name(), errors(), failures(), and
  timestamp() methods return const references. The obsolete
  add_test() method has been removed.

The unit tests have been updated to conform to the new interfaces.
Reviwed GTestCase class.

The name(), message(), type(), kind(), passed() and duration() methods now all return a const reference.