Adjust expected test output

Adjust expected test output for recent changes in libxml2.
Add comment about known-to-fail test

tests/general/bug-182.xsl requires a recent version of libxml2.
Add test case for XPath function args in error case

Add the test case from Jan Pokorný to the test suite. This test is known
to fail with libxml2 2.9.1 and below but fixed in libxml2 Git.