Fix doctests test path

This is a port of astropy/astropy#2518 to astropy-helpers instead of the main astropy repo.  Info below is taked from astropy/astropy#2518 :+1: 


While testing for astropy/astropy#2512 and astropy/astropy#2513 I noticed that the docs-path option to python setup.py test is not respected. This is because it uses a relative path even when the docs build happens in a temporary directory that doesn't include the sphinx source. So this just locks in the absolute path at the time the test command is triggered, rather then when the tests are actually running (which is in a separate process).

It also clarifies the help information for the -t option, which I realized interacts in a confusing way with the docs-path description.