'If __all__ is present, only collect the tasks it specifies'

def test_explicit_discovery(self):

    explicit = fabfile('explicit_fabfile.py')
    with path_prefix(explicit):
        (docs, funcs) = load_fabfile(explicit)
        eq_(len(funcs), 1)
        ok_(('foo' in funcs))
        ok_(('bar' not in funcs))
