Failing expectThat does not fail AsynchronousDeferredRunTest

[Jonathan Lange]

expectThat won't fail tests AsynchronousDeferredRunTest

Minimal example:

```python
from testtools import TestCase
from testtools.deferredruntest import AsynchronousDeferredRunTest
from testtools.matchers import Is

class FooTests(TestCase):

    run_tests_with = AsynchronousDeferredRunTest

    def test_something(self):
        self.expectThat(object(), Is(object()))

if __name__ == '__main__':
    import unittest
    unittest.main()
```

This succeed, where it should fail.