Fix the Jenkins test run by skipping the negative testcases earlier

The problem is that lib.libc.locale.t_io:bad_big5_wprintf was printing out
illegal Unicode characters, which causes XML parsers to bail immediately, e.g.

% kyua report-junit > ~/report.junit
% python2 -c 'import xml.dom.minidom as md; md.parse("/home/ngie/report.junit")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python2.7/xml/dom/minidom.py", line 1918, in parse
    return expatbuilder.parse(file)
  File "/usr/local/lib/python2.7/xml/dom/expatbuilder.py", line 924, in parse
    result = builder.parseFile(fp)
  File "/usr/local/lib/python2.7/xml/dom/expatbuilder.py", line 207, in parseFile
    parser.Parse(buffer, 0)
xml.parsers.expat.ExpatError: not well-formed (invalid token): line 27137, column 13