Add migration with data test for migration 151

Before the 151 migration test is run, _prerun_151 populates the task_log
table with generated date and time in string format for period_beginning and
period_ending. The _check_151 function checks that the migration from string
to DateTime worked as expected.

Partially implements: blueprint migration-testing-with-data

Change-Id: I6d354741a75804bc490ffefde04c5925dd1a2a90
Merge "Add migration with data test for migration 151"