Added a type alias for the node_type.
Fixed that todo for the node type alias in the Quadtree_Tests.ObjectMaxWorks test.
Made use of some more using declarations to make the test code look vaguely cleaner.
These functions in the Quadtree weren't even implemented so there isn't a need for them.
Tests are in respective folders with a consistent naming scheme.

The name of each test file will now be _Tests and the file will be
located in the same subdirectory (server/, common/, client/) as the
file-to-test is located in.