some cleanup in io/ascii/tests

Some of the tests still had import ... as asciitable as a heritage
from the old asciitable package. Similarly, there was some
require_numpy from times when asciitable worked without numpy.
Nothing important, but hey, why not fix that when I am sitting at a
bus station anyway.
Merge pull request #2365 from hamogu/cleanupascii

Change "io.ascii as asciitable" imports to ascii