Add a test of interleaving reads and writes.
Add a test of interleaving reads and writes.

* subversion/test/libsvn_subr/spillbuf-test.c:
  (test_spillbuf_interleaving): interleave a series of writes and reads to
    ensure the data comes out in the proper order and content.
  (test_funcs): add new test


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1173162 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion