test_utils_network.py: skip test when netifaces is not available

netifaces is an external library, that is not trivially installable by
pip (requires a C compiler).  Let's make the test conditional to the
presence of the netifaces library.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/unit/test_utils_network.py

-import netifaces
 import socket
 import unittest
 
@@ -7,6 +6,12 @@ try:
 except ImportError:
     import mock
 
+try:
+    import netifaces
+    HAS_NETIFACES = True
+except ImportError:
+    HAS_NETIFACES = False
+
 from avocado.utils import network
 
 
@@ -52,6 +57,8 @@ def get_all_local_addrs():
 
 class FreePort(unittest.TestCase):
 
+    @unittest.skipUnless(HAS_NETIFACES,
+                         "netifaces library not available")
     def test_is_port_free(self):
         port = network.find_free_port(sequent=False)
         self.assertTrue(network.is_port_free(port, "localhost"))