Add a basic skiplist test.  Based on test program posted to PR 56654 by Takashi Sato.

Submitted By: Takashi Sato, covener
Committed By: covener



git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1604596 13f79535-47bb-0310-9956-ffa450edef68