add and use a login.defs.test with CREATE_HOME set

add and use a login.defs.test with CREATE_HOME set

I suspect this is not a big deal, and most distributions just ship their own
version verbatim like debian/login.defs. But if there is a distro - or even a
person - using this as is from upstream, then we dont' want to break them. So
let's undo this and use an etc/login.defs.test for the testing if needed.

Changelog: 01/13: move etc/login.defs.test to tests/system/etc/login.defs per
                  suggestion.

Signed-off-by: Serge Hallyn <serge@hallyn.com>