Tests: fix RunCMake.CPack_DEB.EXTRA on Debian non-Linux OSes

The generated path with the packages uses $CPACK_TOPLEVEL_TAG, which
by default is $CPACK_SYSTEM_NAME, thus the OS name.

To make the expected stderr match also non-Linux OSes, accept any
non-slash characters in place of "Linux", so it works also on other
Debian OSes (e.g. Debian/Hurd).