utils.token: Use time.time to generate the timestamps

Use `time.time()` rather than `timezone.now()` for generating the unix
timestamps.  This avoids conversion between year-month-day-hh-mm-ss
formatted timestamp vs. unix timestamp and is therefore simpler and more
robust.

Add a test case for this too and amend test_token_endpoint, since it
used to mock timezone.now, but now it needs to mock time.time.

oidc_provider/tests/test_utils.py

@@ -1,6 +1,12 @@
+import time
+from datetime import datetime
+
 from django.test import TestCase
+from django.utils import timezone
 
 from oidc_provider.lib.utils.common import get_issuer
+from oidc_provider.lib.utils.token import create_id_token
+from oidc_provider.tests.app.utils import create_fake_user
 
 
 class Request(object):
@@ -42,3 +48,31 @@ class CommonTest(TestCase):
         self.assertEqual(get_issuer(site_url='http://127.0.0.1:9000',
                                     request=request),
                          'http://127.0.0.1:9000/openid')
+
+
+def timestamp_to_datetime(timestamp):
+    tz = timezone.get_current_timezone()
+    return datetime.fromtimestamp(timestamp, tz=tz)
+
+
+class TokenTest(TestCase):
+    def setUp(self):
+        self.user = create_fake_user()
+
+    def test_create_id_token(self):
+        start_time = int(time.time())
+        login_timestamp = start_time - 1234
+        self.user.last_login = timestamp_to_datetime(login_timestamp)
+        id_token_data = create_id_token(self.user, aud='test-aud')
+        iat = id_token_data['iat']
+        self.assertEqual(type(iat), int)
+        self.assertGreaterEqual(iat, start_time)
+        self.assertLessEqual(iat - start_time, 5)  # Can't take more than 5 s
+        self.assertEqual(id_token_data, {
+            'aud': 'test-aud',
+            'auth_time': login_timestamp,
+            'exp': iat + 600,
+            'iat': iat,
+            'iss': 'http://localhost:8000/openid',
+            'sub': str(self.user.id),
+        })