slapos_erp5: (test) pin Datetime to prevent flakly failures

  After the 25 of the month, the simulation generate extra SPL for discounts,
  pin DateTime to have stable assertions regardless the date the test runs.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

02:46:41

failed SlapOS.Eggs.UnitTest-Master.Python2

00:29:22

failed SlapOS.Eggs.UnitTest-Master.Python3

00:51:09

failed SlapOS-Master.UnitTest-Master

03:07:37

passed SlapOS.Eggs.UnitTest-Master.Python2

00:27:49

failed SlapOS.Eggs.UnitTest-Master.Python3

00:47:57