Extend the test "txn list" list test to still return the correct list
after some txn got aborted.  Covers svn_fs_purge_txn as well now.

* subversion/tests/libsvn_fs/fs-test.c
  (verify_txn_list): Purge txns one-by-one and verify txn list after
                     each step.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1633967 13f79535-47bb-0310-9956-ffa450edef68