Every CPack generator sets default CPACK_PACKAGING_INSTALL_PREFIX
variable value differently so test framework should support
that correctly - previous version expected RPM/Deb /usr default and
removed it for Archive packagers as the default there is /.
* In older version "%dir %attr(-, root, root) foo" would put "%dir foo"
in the final spec file.
* Also added comment to describe this and advise not not to add trailing
slashes to directories in USER_FILELIST.
* Includes test in RunCMake CPack which now passes.