test: assert certificate is created after 36500 days, not 100 years

As we can see today:

AssertionError: ' 2119 ' not found in 'notAfter=Dec 14 00:54:31 2118 GMT\n')

slapos/libnetworkcachetests.py

@@ -657,13 +657,14 @@ class GenerateSignatureScriptTest(unittest.TestCase):
   def test_generate_certificate(self):
     slapos.signature.generateCertificate(self.certificate, self.key,
       self.common_name)
-    today = datetime.date.today()
     result = subprocess.check_output(['openssl', 'x509', '-noout', '-subject',
       '-in', self.certificate], universal_newlines=True)
     self.assertEqual('subject=CN = %s' % self.common_name, result.strip())
     result = subprocess.check_output(['openssl', 'x509', '-noout', '-enddate',
       '-in', self.certificate], universal_newlines=True)
-    self.assertIn(' %s ' % (today.year + 100), result)
+
+    expiration_date = datetime.date.today() + datetime.timedelta(days=36500)
+    self.assertIn(' %s ' % expiration_date.year, result)
 
   def test_generate_key_exists(self):
     with tempfile.NamedTemporaryFile() as key: