gdb/cli: fixes to newly added "list ." command

gdb/cli: fixes to newly added "list ." command

After the series that added this command was pushed, Pedro mentioned
that the news description could easily be misinterpreted, as well as
some code and test improvements that should be made.

While fixing the test, I realized that code repetition wasn't
happening as it should, so I took care of that too.

Approved-By: Andrew Burgess <aburgess@redhat.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>