Add a test for the new spillbuf reader.
Add a test for the new spillbuf reader.

* subversion/tests/libsvn_subr/spillbuf-test.c:
  (test_spillbuf_reader): new test
  (test_funcs): add the new test

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