Test all doubly_linked_list.c functions.

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Move the `DLNode` struct from global to static scope.
	-Redefine `createDLNode()` and `freeDLNode()` as static, and
	move the relevant documentation from `doubly_linked_list.h` to
	`doubly_linked_list.c`.
	-Define `printDLList()`.

data_structures/doubly_linked_list/src/test.c
	-Add statements to `testDLList()` to test various functions
	inside `doubly_linked_list.c`.