TST/PKG: Remove definition pandas.util.testing.slow
And just use `pytest.mark.slow` where needed. Defining the fixture at the top-level there causes problems for projects that import pandas and use the `--strict` flag for pytest.

```bash
$ echo "import pandas" > test_pandas.py
$ pytest test_pandas.py --strict

====================================================== ERRORS ======================================================
_________________________________________ ERROR collecting test_pandas.py __________________________________________
test_pandas.py:1: in <module>
    import pandas
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/__init__.py:60: in <module>
    import pandas.testing
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/testing.py:7: in <module>
    from pandas.util.testing import (
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/util/testing.py:55: in <module>
    slow = pytest.mark.slow
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/_pytest/mark.py:251: in __getattr__
    self._check(name)
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/_pytest/mark.py:266: in _check
    raise AttributeError("%r not a registered marker" % (name,))
E   AttributeError: 'slow' not a registered marker
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
============================================= 1 error in 1.29 seconds ==============================================
```

By replacing usages of `@slow` with `@pytest.mark.slow`, the error is not raised.

The alternative is to register a setuptools entrypoint that exposes the slow marker. I don't think this is desirable, since AFAIK, there's no namespacing on pytest plugins. It doesn't make since for pandas to claim that, since we aren't a pytest plugin really.

I'm marking this for 0.21.0, since it's possible people were using the `@slow` decorator in their own tests. I'll do some searching.

xref https://github.com/pandas-dev/pandas/issues/16680#issuecomment-312383618 and https://github.com/pandas-dev/pandas/issues/16797

cc @koffie