Fixed a bug in InstrusiveList::back

InstrusiveList::back was returning the sentinal value (nullptr) at the
back of the list instead of the last element in the list as expected.
This has been corrected.
Added a testing mechanism

Added a Test class which contains a collection of test entries. Test
entries are meant to test fine grained class behavior; a Test is a
grouping of these.

Added a Test for IntrusiveList.