Flexmock doesn't work correctly with unittest.expectedFailure

[ollipa]

Flexmock doesn't work correctly with unittest.expectedFailure decorator. If a test is marked with expectedFailure and flexmock assertions fail, the test fails due to unexpected success.

https://docs.python.org/3/library/unittest.html#unittest.expectedFailure

We should hook flexmock into addUnexpectedSuccess and addExpectedFailure.

Test case to reproduce the issue:

import unittest

from flexmock import flexmock


class TestExpectedFailure(unittest.TestCase):
    @unittest.expectedFailure
    def test_expected_failure(self):
        flexmock().should_receive("failed").once()

Running the above code produces the following output:

> python -m unittest tests/test_unittest.py
u
----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (unexpected successes=1)