tests/test-xmp-broken: relax check for XML parsing error message.

Fixes:

    ======================================================================
    FAIL: test-xmp-broken.test.test_no_verbatim
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/usr/lib/python2.7/dist-packages/nose/case.py", line 197, in runTest
        self.test(*self.arg)
      File ".../pdf2djvu/tests/test-xmp-broken.py", line 30, in test_no_verbatim
        r.assert_(stderr='XMP metadata error: XMP Toolkit error 201: XML parsing failure\n')
      File ".../pdf2djvu/tests/tools.py", line 98, in assert_
        assert_multi_line_equal(self.stderr, stderr)
    AssertionError: 'XMP metadata error: XMP Toolkit error 201: Error in XMLValidator\n' != 'XMP metadata error: XMP Toolkit error 201: XML parsing failure\n'
    - XMP metadata error: XMP Toolkit error 201: Error in XMLValidator
    ?                                            ---------   ^  ^^^^^
    + XMP metadata error: XMP Toolkit error 201: XML parsing failure
    ?                                               ^^ +++++++++ ^ +

    -------------------- >> begin captured stdout << ---------------------
    $ ../pdf2djvu -q .../pdf2djvu/tests/test-xmp-broken.pdf -o .../pdf2djvu/tests/test-xmp-broken.djvu --version
    $ ../pdf2djvu -q .../pdf2djvu/tests/test-xmp-broken.pdf -o .../pdf2djvu/tests/test-xmp-broken.djvu

    --------------------- >> end captured stdout << ----------------------

with Exiv2 >= 0.27.5.

Exiv2/exiv2@411f1af067ca8e07
#149

tests/test-xmp-broken.py

@@ -13,6 +13,8 @@
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 # General Public License for more details.
 
+import re
+
 from tools import (
     assert_regex,
     case,
@@ -27,7 +29,7 @@ def test_verbatim(self):
     def test_no_verbatim(self):
         self.require_feature('Exiv2')
         r = self.pdf2djvu()
-        r.assert_(stderr='XMP metadata error: XMP Toolkit error 201: XML parsing failure\n')
+        r.assert_(stderr=re.compile(r'\AXMP metadata error: XMP Toolkit error 201: .*\n\Z'))
         xmp = self.extract_xmp()
         assert_regex(xmp, '<broken')