Stop rewriting the shared 'authz' file on every creation of a sandbox in our
testsuite when running the tests parallel.

Rewriting a shared file is just not an option when running the tests in
parallel even when you do this 'atomically' you lock the path for some time
on Windows, so you get test failures.

The main.py code explicitly documents this, but somehow this code got
duplicated here without any of the warnings.

This fixes running the httpd tests in parallel on Windows.

* subversion/tests/cmdline/svntest/sandbox.py
  (_set_name): Stay away from the authz file when running the tests parallel.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552064 13f79535-47bb-0310-9956-ffa450edef68