Ensure unnamed test group categories are separated from previous.
* doc/autoconf.texi (Writing Testsuites) <AT_BANNER>: Update
description.
* lib/autotest/general.m4 (AT_INIT) <at_fn_banner>: Set banner
to single space, not empty line, once printed. For empty
banners, print a single empty line to separate them from a
previous test group category.
* tests/autotest.at (Banners): Insert another test group; adjust
tests accordingly. Extend test to cover semantic change.
* NEWS: Update.
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>