Mock auth_end_time in test_shell.test_download

If we don't we are getting an error like this under py3:

TypeError: unsupported operand type(s) for /: 'float' and 'MagicMock'

Change-Id: If5a6947757297354e6b81fc45f011cc2921d609f
Merge "Mock auth_end_time in test_shell.test_download"