selftests/functional/test_basic.py: skip tests on missing lxml library

This is similar to the xunit unittest changes in 353fed04, but now
applied to the functional tests which happen to use the same features.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/functional/test_basic.py

@@ -20,7 +20,12 @@ try:
 except:
     from BytesIO import BytesIO
 
-from lxml import etree
+try:
+    from lxml import etree
+    SCHEMA_CAPABLE = True
+except ImportError:
+    SCHEMA_CAPABLE = False
+
 from six import iteritems
 from six.moves import xrange as range
 
@@ -1144,6 +1149,8 @@ class ParseXMLError(Exception):
 
 class PluginsXunitTest(AbsPluginsTest, unittest.TestCase):
 
+    @unittest.skipUnless(SCHEMA_CAPABLE,
+                         'Unable to validate schema due to missing lxml.etree library')
     def setUp(self):
         self.tmpdir = tempfile.mkdtemp(prefix='avocado_' + __name__)
         junit_xsd = os.path.join(os.path.dirname(__file__),