added test case for Bug 13971 (libxml2 xpath.c bug, but tested here)

* tests/general/Makefile.am, tests/general/bug-141.out,
  tests/general/bug-141.xsl, tests/docs/Makefile.am,
  tests/general/bug-141.xml: added test case for
  Bug 13971 (libxml2 xpath.c bug, but tested here)