Test Order Dependency between parquet tests and internals
I observe this locally (py2 only)

```
pytest pandas/tests/io/test_parquet.py::TestParquetPyArrow::test_basic  pandas/tests/internals/test_internals.py::test_deprecated_fastpath --tb=no -v
=========================================================== test session starts ============================================================
platform darwin -- Python 2.7.14, pytest-3.3.2, py-1.5.2, pluggy-0.6.0 -- /Users/taugspurger/miniconda3/envs/pandas-ip2/bin/python
cachedir: .cache
rootdir: /Users/taugspurger/sandbox/pandas-ip-2/pandas, inifile: setup.cfg
plugins: xdist-1.22.0, forked-0.2, hypothesis-3.38.5
collected 2 items

pandas/tests/io/test_parquet.py::TestParquetPyArrow::test_basic PASSED                                                               [ 50%]
pandas/tests/internals/test_internals.py::test_deprecated_fastpath FAILED                                                            [100%]

==================================================== 1 failed, 1 passed in 0.35 seconds ====================================================
```

We apparently have an environment variable for controlling deprecation warnings.

```
PANDAS_TESTING_MODE=deprecate pytest pandas/tests/io/test_parquet.py::TestParquetPyArrow::test_basic  panda
s/tests/internals/test_internals.py::test_deprecated_fastpath --tb=no -v
=========================================================== test session starts ============================================================
platform darwin -- Python 2.7.14, pytest-3.3.2, py-1.5.2, pluggy-0.6.0 -- /Users/taugspurger/miniconda3/envs/pandas-ip2/bin/python
cachedir: .cache
rootdir: /Users/taugspurger/sandbox/pandas-ip-2/pandas, inifile: setup.cfg
plugins: xdist-1.22.0, forked-0.2, hypothesis-3.38.5
collected 2 items

pandas/tests/io/test_parquet.py::TestParquetPyArrow::test_basic PASSED                                                               [ 50%]
pandas/tests/internals/test_internals.py::test_deprecated_fastpath PASSED                                                            [100%]

========================================================= 2 passed in 0.13 seconds =========================================================
 ```

It'd be nice if the tests didn't rely on this.