Consider moving tests/ outside the pandas module

#### Problem description

Hi.
After a `pip install pandas` the `lib/site-packages/pands/tests/` includes a lot of testing code which is definitely not relevant for me and many other end users of pandas.
This bloats the installation and makes installation slower.
I'm working on packaging a python environment to distribute with a preinstalled set of modules and application and there are too many popular 3rd-party modules which include unneeded test code, like `numpy`, `IPython` `jupyterlab`, etc, which needs to be striped to keep the package size down. I'll be reporting issues to these projects as well.

#### Suggestion

Therefore, my suggestion is to keep the `pandas` module streamlined, and move the tests out. Perhaps create a `pandas-unittests` module if people are interested in it, or just expect users to checkout the code. Another possibility would be to skip packaging the `tests` folder and `conftest.py` when creating packages to upload to `pypi.org`.

Since you are now almost releasing 1.0 it might be a bit short notice to include this is such a big release. But for the next major release, it could work.

Thank you very much for your attention.