EA.values warning test
xref #20735 (doesn't close)

Here's a local "test" for it.

```
pytest pandas/tests/extension/test_values.py -k test_no_values
==================================================================== test session starts ====================================================================
platform darwin -- Python 3.7.0b3, pytest-3.5.0, py-1.5.3, pluggy-0.6.0
rootdir: /Users/taugspurger/sandbox/pandas, inifile: setup.cfg
plugins: xdist-1.22.2, forked-0.2, cov-2.5.1
collected 9 items / 8 deselected

pandas/tests/extension/test_values.py F                                                                                                               [100%]

========================================================================= FAILURES ==========================================================================
______________________________________________________ TestBaseInterfaceTests.test_no_values_attribute ______________________________________________________

self = <pandas.tests.extension.test_values.TestBaseInterfaceTests object at 0x112ed4748>, data = JSONArary([{'A': [1, 2], 'B': [3, 4]}])

    def test_no_values_attribute(self, data):
        # GH-20735
        # Currently, pandas has places where we accepts Union[ndarray, EA]
        # but in practice expect an ndarray, since we get `data.values.dtype`.
        # This warns users to avoid using a `.values` attribute that is not
        # an ndarray-like
        if hasattr(data, 'values') and not hasattr(data.values, 'dtype'):
            msg = ("ExtensionArray contains a 'values' attribute that does "
                   "not have a dtype attribute. This may cause issues in "
                   "pandas' internals.")
>           raise ValueError(msg)
E           ValueError: ExtensionArray contains a 'values' attribute that does not have a dtype attribute. This may cause issues in pandas' internals.

pandas/tests/extension/base/interface.py:64: ValueError
========================================================== 1 failed, 8 deselected in 0.12 seconds ===========================================================
```

What, if anything, should we include in pandas to test this? I'm fine with the current PR (no tests), since it's just test code.

Edit: ignore what I said about testing. Added one that calls `super()` inside a `raises` context manager. It's not too bad.