Update test_ioloop.py

[gkrimer]

Remove return_vaue=mock.Mock()

It should be return_value=mock.Mock() (fix typo), but even this is likely not needed as the return value of a mock object is another mock.

[coveralls]

Coverage remained the same at ?% when pulling d4b77ed on gkrimer:patch-1 into 298e48d on giampaolo:master.