assert_raises_such_that should assert that the lambda's evaluation is None or is True

[asottile]

import testify as T

class Regression(T.TestCase):

    # XXX: if I had an expected failure decorator I'd use it here
    def test(self):
        T.assert_raises_such_that(ValueError, lambda e: e.args == ('foo',)):
            raise ValueError('bar')

[bukzor]

There's prior art in our assertions.wait_for that does what anthony expects:

            result = assertion()
            if result is not None:
                # A value was returned. Was it truthy?
                assert result, 'Not truthy: %r' % result

[bukzor]

@asottile This should be a quick fix for you some day when you have downtime.