Re-toggle build units, examples & tests for this branch
tests/CMakeLists.txt: Move config of units to their own scripts

  Rename cmake vars:
NOM_BUILD_PTREE_JSON_TESTS -> NOM_BUILD_SERIALIZERS_JSON_TESTS
NOM_BUILD_PTREE_XML_TESTS -> NOM_BUILD_SERIALIZERS_XML_TESTS

* Graphics related unit test configuration has been moved to
tests/graphics/CMakeLists.txt.

* Math related unit test configuration has been moved to
tests/math/CMakeLists.txt.

* System related unit test configuration has been moved to
tests/system/CMakeLists.txt.

* Serializer (JSON, XML) related unit test configuration has been
moved to tests/serializers/CMakeLists.txt.

* Property Tree related unit test configuration has been moved
to tests/ptree/CMakeLists.txt.

* GUI related unit test configuration has been moved to
tests/gui/CMakeLists.txt.