API: add testing functions to public API ?
A lot of other projects that use pandas will (like to) use pandas testing functionality like `assert_frame_equal` in their test suite. Although the pandas testing functions are available in the namespace (#6188), they are not really 'officially' labeled as public API that other projects can use (and rely upon). 
Numpy has a similar submodule  `numpy.testing` (http://docs.scipy.org/doc/numpy/reference/routines.testing.html)

Some things we could do:
- make a selection of the functions in `util.testing` that we want to label as public
- add this list somewhere to the docs
- write docstrings for these public ones (the other could use that as well of course ..)
- add some tests for the public API
- I would also import them into a `pandas.testing` module, so it is this one we can publicly advertise (and users are less tempted to use other non-public functions in the `pandas.util.testing` namespace)
