Provide single base class with all extension tests.
It's slightly annoying to have files like

```python
class TestMissing(base.BaseMissingTests):
    pass

class TestOps(base.BaseOpsTests):
    pass
```

for all the extension tests. It'd be nice to have a single base class. The problem is ensuring names don't clash. Python has a built-in mechansim, name mangling, for doing what we want, but it would involve an API breaking change for tests (there's probably a clever hack that could avoid breaking API).

```python
# test_foo.py
class A:
    def __test_bar(self):
        assert 1

class B:
    def __test_bar(self):
        assert 0


class BaseTests(A, B):
    pass



class TestIt(BaseTests):
    pass
```

and a `pytest.ini`

```
[pytest]
python_functions=_*__test*
```

I don't think we should, am curious to hear others' thoughts. 