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

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion