Adds interface tests for timeutils

We were originally testing the return value of timeutils.parse_strtime
in our unit tests. Instead of interleaving tests for an external library
into Keystone tests I gave them a place of their own.

Change-Id: I2601b1198ca841c71a7590c7b00b11bcede4d4a9

keystone/tests/unit/external/README.rst

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+This directory contains interface tests for external libraries. The goal
+is not to test every possible path through a library's code and get 100%
+coverage. It's to give us a level of confidence that their general interface
+remains the same through version upgrades.
+
+This gives us a place to put these tests without having to litter our
+own tests with assertions that are not directly related to the code
+under test. The expectations for the external library are all in one
+place so it makes it easier for us to find out what they are.

keystone/tests/unit/external/test_timeutils.py

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+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+import datetime
+
+from oslo_utils import timeutils
+
+import keystone.tests.unit as tests
+
+
+class TestTimeUtils(tests.BaseTestCase):
+
+    def test_parsing_date_strings_returns_a_datetime(self):
+        example_date_str = '2015-09-23T04:45:37.196621Z'
+        dt = timeutils.parse_strtime(example_date_str, fmt=tests.TIME_FORMAT)
+        self.assertIsInstance(dt, datetime.datetime)
+
+    def test_parsing_invalid_date_strings_raises_a_ValueError(self):
+        example_date_str = ''
+        simple_format = '%Y'
+        self.assertRaises(ValueError,
+                          timeutils.parse_strtime,
+                          example_date_str,
+                          fmt=simple_format)