TaskTest:test_fail() should use asserIstNone

test_fail() in the TasksTest class of tests/unit/test_domain.py uses
assertEqual when testing for None; assertIsNone is more appropriate
here.

This is the kind of thing that would be automatically picked up
by change Ic115342605c472f3a5d255aa570ecb60175ca087

Change-Id: I900334182f056f8b3656339b5bad0b60b334e7dd
Merge "TaskTest:test_fail() should use asserIstNone"