Cleanup test infrastructure creation a bit.

* subversion/tests/libsvn_wc/db-test.c
  (create_open): Remove tree before creating new data.

* subversion/tests/libsvn_wc/utils.c
  (svn_test__create_fake_wc): Remove single file DB delete right before
    creating the new database.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558566 13f79535-47bb-0310-9956-ffa450edef68