Following up on r1552079, fix an ordering problem by introducing a
helper method.

* subversion/tests/cmdline/svntest/sandbox.py
  (build): Make sure the repository is accessible before the checkout
    by calling the authz change a bit earlier.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552091 13f79535-47bb-0310-9956-ffa450edef68