slapos_slap_tool: Add test to call API Twice

  'since' should be unmutable.
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

00:13:05

failed SlapOS.Eggs.UnitTest-Master.Python3

00:10:24

canceled SlapOS.Eggs.UnitTest-Master.Python2

failed SlapOS.Eggs.UnitTest-Master.Python2

00:12:19

failed SlapOS.Eggs.UnitTest-Master.Python3

00:10:53