Fix python setup.py test to run doctests.

[pelson]

By defining the tests to be in cf_units/tests we are excluding doctests found in cf_units when running python setup.py test (which is the case in our CI). Locally, I personally run pytest cf_units which actually fails with a few doctests.

This changes the config so that python setup.py test identifies the doctests in cf_units, and fixes the few tests that were failing.

For the record, it took me a deal of time to figure out what was going on. I was making sure that setup.cfg could actually handle pytest aliases and addopts, started down the line of implementing a pytest.ini to see if that was the issue. A couple of issues that came up (pytest-dev/pytest-runner#18, astropy/astropy#6513) were red-herrings - this was simply fixed by the fact that the testpaths were pointing to the test folder, and not the whole source folder!

[pelson]

Hopefully coveralls will come back and say that coverage has increased (markedly). This will be the thing that gives us confidence in the future that this hasn't regressed (because a regression would mean that we run fewer tests).

[coveralls]

Coverage increased (+1.3%) to 89.568% when pulling 0b3fbcf on pelson:doctest into 502d014 on SciTools:master.

[coveralls]

Coverage increased (+1.3%) to 89.568% when pulling 0b3fbcf on pelson:doctest into 502d014 on SciTools:master.

[coveralls]

Coverage increased (+1.3%) to 89.568% when pulling 0b3fbcf on pelson:doctest into 502d014 on SciTools:master.

[bjlittle]

Awesome, thanks @pelson 👍