Reset namespace for testing config docs

Currently doctesting has been switched off for `ScienceState` related docs, but ideally anything that suppose to work for the users should be tested. To make this happen we need to reset the namespace while testing, I'm certain there is a clever way to do it.

Bottom line, those docs should be tested by the time we ship it to the users, if nothing else by brute force removing the modules prior in a `testsetup` directive, etc.

See discussion here: https://github.com/astropy/astropy/pull/8517#discussion_r289228267