Close file handles asap - even in test code.  Windows has trouble deleting
folders with open files in it.

* subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
  (upgrade_txns_to_log_addressing): Use an iterpool inside our data 
                                    manipulation loops to have more control
                                    over open file handles.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547099 13f79535-47bb-0310-9956-ffa450edef68