TST: move conftest.py to top-level
xref #15341 
@jreback The concerns of before regarding `pd.test()` (the reason conftest.py was not in the top level), are those solved / not needed anymore ? @jorisvandenbossche so the issue was that when installed, if ``conftest.py`` is at the top-level (and not below pandas), then its not installed; this can be resolved by by actually including it as a data file in ``setup.py``, but then need to have to explicity tell ``pytest`` . This *only* matters for ``pandas.test()``; so we *could* explicity path it in there.

But this is just simpler. In any even I made the ``BUILD_TEST`` build now run ``pandas.test()`` so this is tested anyhow. FYI I reverted my original commit: 59b88ab6123338e1dfcc0f77dfb3e5c4c511889a  For what reason did you revert?

Personally, it doesn't really matter to me where `conftest.py` goes. It is just a choice when putting it top-level that `pd.test()` cannot specify the custom markers. Which is not the biggest problem I think. But I also don't mind it being inside pandas (it is not visible to users, so not really a problem) @jorisvandenbossche well, it IS a problem in that in reality ``conftest.py`` will have more and more things. This is a central repository for fixtures and such. If pandas had been built from the ground up with ``pytest``, we would have a whole set of ``conftest.py``. So its not a problem per-se, just not very clean (and not very typical). OK, so we agree to just keep it in pandas? 
That was not fully clear to me that you were OK with that, so that was the only reason of my question above! yeah not sure what to do with it really.