Switch diff-diff3-test.c to using our current io apis instead of the apr ones
that need the binary flag to avoid platform differences.

* subversion/tests/libsvn_diff/diff-diff3-test.c
  (make_random_file,
   make_file,
   three_way_merge,
   two_way_diff,
   make_random_merge_file,
   random_trivial_merge,
   random_three_way_merge,
   merge_with_part_already_present): Use svn_io_* instead of apr_file_*.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436723 13f79535-47bb-0310-9956-ffa450edef68