make `assertEllipsis` compatible with py.test's function bases tests

[sallner]

transfered from bitbucket:

Michael Howitz created an issue 2016-12-05
assertEllipsis expects that it can access a fail() method on self. It has to be provided by the class which mixes in the Ellipsis class.
Expectation: Possibility to use assertEllipsis outside unitest.TestCase based tests.