Extend stream unit testing by test code for svn_stream_skip.

* subversion/tests/libsvn_subr/stream-test.c
  (test_stream_seek_file, test_stream_seek_stringbuf, 
   test_stream_seek_translated): replicate seek&read code to test skip()

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/integrate-stream-api-extensions@1072516 13f79535-47bb-0310-9956-ffa450edef68