don't run doctests when -t option given

Before the doctests were added, doing ``python setup.py -t astropy/path/to/test_file.py`` would just run that one file.  But now it runs that file *and* all the doctests.  So this PR changes it to only run the file and not the doctests. It's pretty straightforward, but I figured I should check with @mdboom : the current behavior isn't intentional, is it?