fix: make datetime objects readable

[GangaManoj]

Problem:

Datetime objects in all the old tests in test_issue.py have all been written like datetime.datetime(2019, 3, 4, 12, 0), which isn't very readable.

Fix:

Use get_datetime to write them instead.
eg: Replace datetime.datetime(2019, 3, 4, 12, 0) with get_datetime("2019-03-04 12:00").

[coveralls]

Coverage remained the same at 51.551% when pulling c4defc6 on GangaManoj:make-datetime-objects-readable into d1fe060 on frappe:develop.