libiberty: disable logging of list content for doubly-linked list tests

libiberty: disable logging of list content for doubly-linked list tests

When the doubly-linked list tests were introduced, the tests were
printing the content of the list forward and backward. However, this
printing is not needed outside of debugging, and confuses people because
the output is not only composed of PASS: lines.

This patch disables the printing of the list content by default. If
one wants to re-enable it for debugging, he can set the macro DUMP_LIST
to 1.

libiberty/ChangeLog:

	* testsuite/test-doubly-linked-list.c: disable debug logging on
	stdout.