Make the "reopen txn" test slightly more meaningful and use it to cover
svn_fs_txn_root_name() as well.

* subversion/tests/libsvn_fs/fs-test.c
  (reopen_trivial_transaction): Create 3 txns such that SVN has at least
                                a chance to return the wrong one. Verify
                                that it picked the correct one.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1633966 13f79535-47bb-0310-9956-ffa450edef68