tito.props: Add test_version_suffix = ^9999

The caret symbol (‘^’) is used before a version component which must sort later than any non-caret component. It’s especially useful here to ensure that development builds of the rhel-x.y branches (which always stay at Release: 1) are sorter higher than the builds in CentOS/RHEL repositories (whose Release is continually incremented).

For example, if dnf build from c10s, rhel-10-main, or the upstream rhel-10.2 branch of dnf requires libdnf >= 0.73.1-7, then it’s convenient if our upstream libdnf rhel-10.2 builds satisfy that requirement, e.g. by having version 0.73.1^9999-1.