Follow-up to r1633967: Fix test failure with BDB.

This restores the original verify_txn_list and introduces a separate one
for the purge test.  FSFS and FSX backends don't distinguish between
"alive" and "dead" transactions but BDB does.  Use a sub-routine to
handle those differences generically.  As a side-effect, svn_fs_purge_txn
gets better test coverage.

* subversion/tests/libsvn_fs/fs-test.c
  (verify_txn_list): Revert to pre-r1633967 state.
  (cleanup_txn): New utility function.
  (purge_txn_test): The new txn purging test.
  (test_funcs): Register the new test.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1634310 13f79535-47bb-0310-9956-ffa450edef68