Following up on r1552065, resolve an old todo by moving the creation of the
authz file to the build operation of the sandbox. This avoids writing the
file twice for every sandbox.

Also read the file before replacing it. This makes the parallel behavior more
stable on Windows while I'm trying to find out why
svntest.main.options.parallel is 0 on Windows even when running tests
in parallel.

* subversion/tests/cmdline/svntest/sandbox.py
  (_set_name): Move some code from here to...
  (build): ... here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552079 13f79535-47bb-0310-9956-ffa450edef68