Improve skiplist tests.

git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1611110 13f79535-47bb-0310-9956-ffa450edef68
Use apr_skiplist_add() instead of apr_skiplist_insert() to merge skiplists
since doublons need to be merged too.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1611117 13f79535-47bb-0310-9956-ffa450edef68
Call free() on the skiplist structure in apr_skiplist_destroy() if it was
apr_skiplist_init()ialized without a pool (ie. malloc()ed).


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1611120 13f79535-47bb-0310-9956-ffa450edef68
Let apr_skiplist_find_compare() handle given NULL iterator, and be safe
from NULL returns.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1611125 13f79535-47bb-0310-9956-ffa450edef68