test-keyval: Tweaks to improve list coverage

We have a negative test case for a list index with leading zero.  Add
positive ones.

Tweak the test case for list index greater or equal the number of
elements: test "equal" instead of "greater" to guard against
off-by-one mistakes.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1490014548-15083-2-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
