OPENDJ-2958 Search on wrong dn under cn=config should fails with NoSuchEntry error

Problem is on the underlying MemoryBackend used by the ConfigurationHandler
(and the later being used by the ConfigurationBackend).

MemoryBackend.searchWithSubordinates() should first verify whether the baseObject exists,
and if not, return NO_SUCH_OBJECT.
MemoryBackendTestCase may have caught this if the InternalConnection (actually AbstractConnection.getSingleEntry())
was a lot more basic and avoided doing elaborate checks.