selftests/unit/test_utils_cpu.py: skip tests on older mock versions

Some versions of the unittest mock library cannot easily deal with
(builtin) open().  Let's skip these tests on those platforms, instead
of coming up with cumbersome code to support them on all (ancient)
versions.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/unit/test_utils_cpu.py

@@ -10,6 +10,11 @@ except ImportError:
 from avocado.utils import cpu
 
 
+def recent_mock():
+    major = int(mock.__version__.split('.')[0])
+    return major >= 2
+
+
 class Cpu(unittest.TestCase):
 
     @staticmethod
@@ -19,6 +24,8 @@ class Cpu(unittest.TestCase):
         file_mock.__exit__ = mock.Mock()
         return file_mock
 
+    @unittest.skipUnless(recent_mock(),
+                         "mock library version cannot (easily) patch open()")
     def test_s390x(self):
         s390x = u"""vendor_id       : IBM/S390
 # processors    : 2
@@ -87,6 +94,8 @@ cpu MHz static  : 5504
                             return_value=self._get_file_mock(s390x_2)):
                 self.assertEqual(len(cpu.cpu_online_list()), 4)
 
+    @unittest.skipUnless(recent_mock(),
+                         "mock library version cannot (easily) patch open()")
     def test_x86_64(self):
         x86_64 = u"""processor	: 0
 vendor_id	: GenuineIntel