TST: test for GH4633, bool(obj) behavior

closes #4633

- this PR is just tests, currently under discussion
- currently ``bool(pandas_obj)`` returns ``not empty``, that is ``True`` if the pandas_obj has a non-zero length

exception is a single-element Series that will return the ``bool``-ness of that element

```
In [17]: s = Series(randn(4))

In [18]: df = DataFrame(randn(10,2))

In [19]: s_empty = Series()

In [20]: df_empty = DataFrame()

In [5]: bool(s)
Out[5]: True

In [6]: bool(s_empty)
Out[6]: False

In [7]: bool(df)
Out[7]: True

In [8]: bool(df_empty)
Out[8]: False

In [9]: bool(Series([True]))
Out[9]: True

In [10]: bool(Series([False]))
Out[10]: False
```