Follow-up to r1537358: Fix - hopefully - tests on Windows.

* subversion/tests/libsvn_repos/repos-test.c
  (test_config_pool): prepend drive letter with a root slash

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537364 13f79535-47bb-0310-9956-ffa450edef68