intermittent vmtests errors due to clock drift

The following unittest has a spurrious failure due to time drift between when timestamped files are created, and unit test validation tests/unittests/test_commands_collect_logs.py: TestCreateTar.test_create_log_tarfile_copies_configured_logs

This results in occasional jenkins CI failures like the following https://jenkins.ubuntu.com/server/job/curtin-ci/3726/nodes=metal-s390x/consoleFull

We need to find a way to have to unit test either:
 1. mock the time when files are created to avoid mismatch when verifying
 2. allow unit test to verify allowing for a permissable time drift between creation and validation
 3. force the test to emit a known stable file name

12:25:03 FAIL: create_log_tarfile copies configured log_file and post_files.
12:25:03 ----------------------------------------------------------------------
12:25:03 Traceback (most recent call last):
12:25:03 File "/var/lib/jenkins/servers/server/workspace/curtin-ci/nodes/metal-s390x/curtin-3726/tests/unittests/test_commands_collect_logs.py", line 307, in test_create_log_tarfile_copies_configured_logs
12:25:03 mock.call(log1, self.tardir), self.m_copy.call_args_list)
12:25:03 AssertionError: call('/tmp/curtin-ci-TestCreateTar.EVBmg8/some.log', 'curtin-logs-2019-10-07-18-24') not found in [call('/tmp/curtin-ci-TestCreateTar.EVBmg8/some.log', 'curtin-logs-2019-10-07-18-25'),